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POPULAR 



PHYSICAL ASTRONOMY; 



OR, AN EXPOSITION OF 



REMARKABLE CELESTIAL PHENOMENA. 



BY DANIEL VAUGHAN. 



CINCINNATI : 
TRUMAN & SPOFFORD, 

25 WEST FOURTH-STREET. 
1858, 



Entered according to Act of Congress, in the year 1858, by 

DANIEL VAUGHAN ; 

Lithe District Court of the United States, for the Southern 

District of Ohio. 



PREFACE. 



An extensive department of human knowledge has 
been laid open in modern times by a study of the forces 
which pervade nature, and the operations which proceed 
from their action. It has been found that celestial ar- 
rangements depend on the causes which prevail on our 
planet, and that our experience with terrestrial objects is 
sufficient to bring us into communion with the most re- 
mote parts of the creation. From their conformity to 
simple and determinate laws, the phenomena of the 
Heavens are rendered intelligible to the limited powers 
of the human mind, and we are thus enabled to realize 
the greatest advantages from the revelations of the teles- 
cope, and from the several facts which science has placed 
at our disposal. 

Since the time of Sir I. Xewton, the labors of astrono- 
mers and mathematicians have been spent more in perfect- 
ing the work which he commenced than in opening new 
fields of inquiry. The science of Physical Astronomy 
has been much embarrassed and rendered more inacces- 
sible by the great minuteness which Laplace and other 
writers have endeavored to attain, and by their superflu- 
ous use of mathematical formulas. As it has not been 
enriched to a corresponding degree by discoveries, it has 
become less inviting even to the most highly educated 
minds, and its study is almost exclusively confined to the 
universities of Europe, where a few favored individuals 
are permitted to devote to it the amount of time and at- 
tention which so difficult and scientific a pursuit demands. 
The popular sketches of Physical Astronomy sometimes 
introduced into works are too superficial to be of much 
value. 

It has been the object of the author to render this de- 
partment of Astronomy more acceptable to the general 

(ui) 



IV PREFACE. 

reader by pursuing a plan somewhat different from any 
hitherto attempted. He has endeavored to make each 
subject more intelligible by the use of proofs and illustra- 
tions more intimately connected with common experience, 
and he hopes that his contribution to the science will 
render it more calculated to repay the labor of the student. 
The greater part of the original views which he introduces 
have been already laid before scientific men, and have 
been received with as much interest and favor as new 
developments in science generally command. The theory 
of planetary rings, and the stability of satellites, given in 
the fourth and fifth chapters, were made the subject of a 
paper read at the American Scientific Convention for 
1856, and published in the proceedings, page 111. The 
theories of the effects of tidal action, and of the origin of 
solar light, were presented to the British Association for 
the Advancement of Science during the past year. The 
greater part of the chapter on variable stars and the sun's 
spots, was published in the Philosophical Magazine for 
May, 1858. He hopes, therefore, that the misrepresenta- 
tions which his views have received from a few public 
journals will not deter the reader from giving an impar- 
tial examination to the original portions of the present 
work. 

The subjects treated of in the following pages have 
much engaged the author's attention during many years, 
and he accordingly feels more confidence in offering them 
to the public. But he cannot expect to present them in 
so acceptable a form as might be desirable, as the prepara- 
tion of the work occupied only a very inadequate time, 
and was undertaken under very unfavorable circum- 
stances. It is to be hoped, however, that a want of 
literary merit will not detract much from its scientific 
value, and that a reasonable indulgence will be given for 
any defects which it may contain. 



P P U I A E 



PHYSICAL ASTRONOMY 



THE INFLUENCE OF MAGNITUDE ON STABILITY. 



The great diversity of size in natural objects becomes a 
source of admiration to all who first direct their attention to 
the study of the material world. The minute animalcule, 
sporting with myriads of its fellow beings in a drop of water, 
presents a singular contrast with the whale or the elephant ; 
and these animals in turn appear equally insignificant when 
compared with the globe on which we dwell, or the mighty sun 
which controls its destiny. By the revelations of the micro- 
scope, telescopic wonders are rendered more overpowering to 
the imagination ; and the panorama of suns and worlds which 
meets the eye of the astronomer receives a great accession of 
grandeur from the knowledge that a drop of water or a single 
leaf may be a world in itself. On examining the works of Na- 
ture, from the largest to the smallest scale of being, and dis- 
covering the great variety which prevails through her extensive 
realms, it becomes an object of interest to the reflective mind 
to ascertain the several conditions on which these manifold 
forms of existence depend. 

The present inquiry shall be devoted to a consideration of 
physical stability, without which the harmonious career of na- 
ture could not be maintained. All material forms, though per- 
ishable in their character, have generally assigned to them a 
term of existence corresponding to the purposes they are called 
to fulfill, and each enjoys a degree of security in some measure 
proportionate to its importance, or to the part it serves in the 



6 THE INFLUENCE OF MAGNITUDE 

economy of creation. The trembling leaf soon falls a victim to 
the fury of the storm, but our globe is capable of contending 
successfully with all the destructive agencies to which it can be 
exposed during many millions of years. Its rapid march 
through space cannot introduce disorder among its parts, nor 
can the great heat which appears to prevail in the subterranean 
regions ever extend its desolating energy at one time over any 
considerable portion of our habitable lands. The investigations 
of Lagrange and Laplace have shown ths.t the mutual disturb- 
ances between the several planets cannot cause any derange- 
ment of our solar system, and our apprehensions in regard to 
external dangers must vanish when we consider the immense 
distance that separates us from the dominion of other suns. 
During less enlightened ages, comets have too frequently spread 
dismay among mankind, but there seems to be little ground 
for such apprehensions, for so wide is the planetary domain 
that many myriads of these celestial wanderers mayfly through 
it without any serious danger of striking our terrestrial sphere 
or any other member of the solar family. 

The mind which would embark in the investigation of the 
causes that uphold so many suns and worlds, must first be pro- 
vided with a store of facts and principles derived from the ob- 
servation of the course of nature around us. It may be also 
advisable to make the study of the works of art a standing 
point of our inquiries, for man's achievements, though far infe- 
rior to those of nature, are much better understood, and involve 
principles which lie more within the range of human compre- 
hension. Believing that the knowledge which has enabled the 
architect to adorn the Earth with so many monuments of his 
skill may not be unprofitable to the student of the architecture 
of the Heavens, I shall begin with the consideration of a subject 
which is generally regarded as entirely foreign to this depart- 
ment of physical science. 

The degree of firmness or solidity which it is possible to im- 
part to all artificial structures depends not only on the skill of 
the artisan, but also on the passive strength of the materials 
placed at his disposal. In consequence of the cohesion and the 
elasticity of their particles, solid bodies are enabled to with- 



ON STABILITY. 7 

stand, to a certain extent, all the strains to which they are sub- 
jected, and man is thus furnished with the means for construct- 
ing his most imposing works. Of the various materials called 
into requisition by art, iron occupies the most prominent place 
in respect to passive strength. A tun weight might be sus- 
pended by an iron wire not more than one-fifth of an inch in 
diameter ; and a similar wire, if quite free from defects, would 
be capable of supporting about seven miles of its own length 
without being torn asunder. When bars of the same metal 
are exposed to transverse or lateral strains, their strength is ex- 
hibited in a much less degree, and it is proportional to their 
breadth multiplied by the square of the depth and divided by 
their length. 

A bar of cast iron, five feet long and one inch square, would 
be capable of bearing only about five hundred pounds, if sup- 
ported at both ends and loaded at the middle point. By doub- 
ling the length, the breadth and the depth of the bar, we should 
give it four times the strength and eight times the weight it 
previously possessed ; and it may be shown that in similar 
solids the resistance to fracture is proportional to the square of 
any of the linear dimensions, while their own weight is propor- 
tional to the cube of the same quantities. A beam of iron, 500 
feet long and 100 inches square, with its ends resting on two 
supports, would be inevitably broken by its own weight, which 
must exceed 8,000 tons. From this we may infer the impossi- 
bility of extending the size of structures beyond a certain limit; 
and it appears that those of enormous magnitude suffer most 
from the pressure and the inertia of their own parts. The 
most lofty buildings are the most certain victims to the violence 
of an earthquake ; the tallest trees are the first to yield to the 
rage of the tempest ; and, though a small boat may strike a 
rock with impunity, a similar collision would consign the ma- 
jestic ship to inevitable ruin. 

In the animal frame, where such complicated movements are 
inseparable from existence, undue size operates to the greatest 
disadvantage. Whatever be the source of vital power or the, 
intensity which it is capable of attaining, there must be a limit 
to its efficiency, in consequence of the fragility of the solid 



8 THE INFLUENCE OF MAGNITUDE 

frame-work of the system when strained beyond a certain de- 
gree ; and the maximum power of animals of similar organiza- 
tion may therefore be regarded as proportional to the square of 
their linear dimensions. If men attained a stature of five or 
six fathoms, with a corresponding development of bone and mus- 
cle, they might indeed be about thirty-six times as strong as the 
present members of the human race, but they would be encum- 
bered by a weight over two hundred times as great as that of 
our bodies, and accordingly it must be very dangerous and dif- 
ficult for them to move about or to maintain an erect position. 
We may therefore condemn the unnatural fictions of the ancient 
poets, who represent their giants as rivaling in stature the 
tallest trees of the primitive forest. Such unwieldy monsters 
could only walk on the bottoms of seas or deep rivers, where 
the sustaining power of the liquid element almost deprived 
them of weight, but their huge frames would be quite immov- 
able on the land. 

In consequence of the diminution of their weight by the 
presence of the water, the tenants of the deep are permitted to 
attain a far greater size than any terrestrial animal. But, even 
here, an undue size exposes the larger tribes to much incon- 
venience from the inertia of their bodies, and their movements 
display less vigor of action than we should expect to result from 
their amazing strength. The gigantic whale, notwithstanding 
his high organization and the elevated character of his instinct, 
is far from being the lord or the tyrant of the ocean, and he 
falls a victim not only to the ingenuity of man, but also to the 
activity of animals far inferior to him in size and in bodily 
power. 

If the relative force which land animals need for supporting 
the weight of their bodies is much greater than is necessary 
for fishes to propel themselves through the water, it is far in- 
ferior to what the winged tribes must possess for soaring into 
the air. Accordingly, the capability of flying can be conferred 
only on animals of much smaller dimensions than the most 
gigantic tenants of the land and water. Some species of the 
whale, when full grown, are about seventy times as large and 
as heavy as the most gigantic elephant, and the elephant, in 



ON STABILITY. 9 

turn, lias a still more decided superiority in size and weight 
over the greatest bird that traverses the atmosphere. Some of 
the largest birds now living, though possessed of wings, only- 
use them to accelerate their motions on the land, and it is prob- 
able that the same may be said of the colossal winged tribes 
which tenanted the geological world. 

It thus appears that mere mechanical conditions are not with- 
out their influence in determining the characters of life in the 
three departments of our planet, so that the air, notwithstand- 
ing its numerous population of living beings, can never furnish 
such gigantic inhabitants as dwell on the land or in the ocean. 
In consequence of the superior size of man, his relative strength 
is much inferior to that of birds, and this precludes the possi- 
bility of his ever using machinery with success for flying through 
the air. If terrestrial gravity were reduced to one-tenth of its 
present force, and a tenfold increase were given to the air which 
surrounds our globe, artificial flying would no longer be impos- 
sible, and our atmosphere would soon become a highway be- 
tween nations. Such an arrangement, 1 owever, though confer- 
ring some privileges on man, would afford at least a possibility 
of the existence of flying monsters, who might become trouble- 
some enemies to the human race. But we should have far 
more to fear from the depravity of our own species, for, as na- 
tions could then have little means of protection from invaders, 
wars would be attended with the most horrible consequences, 
while crime obtained more chances of impunity. Accordingly, 
though we may envy the enjoyment of the feathered tribes who 
sport over our heads, we have no just cause to be displeased 
with the restraints which have been imposed on human liberty 
by the fixed laws of nature. 

In setting limits to human possibility, it is not to be sup- 
posed that no allowance is made for future improvements in 
the arts. In the execution of all his works, man makes con- 
tinual advances to perfection ; and in every succeeding age he 
is able to leave more stupendous monuments of his skill behind 
him. As his knowledge of the properties of bodies increases, 
he is enabled to derive more advantage from the resources 
which have been placed within his reach ; by studying the 



10 THE INFLUENCE OF. MAGNITUDE 

laws of nature, he obtains a more extensive command of her 
potent agencies, and new inventions ever furnish him with 
more effective means for the accomplishment of his designs. 
The invention of the arch enabled the Romans to give their 
public edifices a scale of magnitude far surpassing what the 
Grecian architecture could attain. By the use of the sus- 
pension bridge during the present age, highways have been 
formed over the largest rivers without destroying their utility 
for the purposes of navigation. The great strength of wrought 
iron tubes, in proportion to their weight, has been recognized 
hj modern art ; and to their use are we indebted for the con- 
struction of the largest vessel that ever braved the fury of the 
waves. 

But no materials placed at man's disposal have a degree of 
strength sufficiently great to afford unlimited scope for archi- 
tectural ambition, and their power of resistance seems taxed 
to the utmost extent in many works of modern times. The 
frequent failures of suspension bridges and other edifices may 
be regarded as warnings to those who are desirous of giving 
structures a greater magnitude than the character of the 
materials will justify. In sounding great depths of the ocean, 
it was long found impossible to prevent the line from being 
broken by a weight sufficient to sink it ; and at last success 
could be only attained by sacrificing a ball at every attempt. 
The chief difficulty in forming a telegraphic communication 
between the Old and New World arises from the liability of the 
cable to break by its own weight while sinking to the bed of the 
ocean. Towers and massive pyramids exhibit their strength 
to the greatest advantage, and man cannot expect to be able 
to give such structures the size which would be fatal to their 
stability. It may be safely asserted, however, that if a pyra- 
mid of granite were fifteen miles high, its base should be 
crushed by the weight it had to bear; end, though giants had 
the power of piling one. mountain on another, they could never 
raise so lofty an edifice from the rocks found at the earth's 
surface. 

There are, indeed, in the laboratory of nature giant forces, 
whose incessant action would be capable of forming such high 



ON STABILITY. 11 

pinnacles on our globe, if the frailty of terrestrial matter put no 
obstacles in their way. Of the water annually evaporated 
from the ocean and conveyed by the winds to many lands, a 
vast quantity falls as snow on the tops of high mountains, 
where the temperature never rises sufficiently high to restore 
it to a liquid state. At great distances from the sea the air is 
so dry that much of the snowy mantle gradually disappears 
by evaporation, and in these localities perpetual snow is found 
considerably above the elevation which temperature would 
prescribe to it under different circumstances. But near the 
ocean the evaporation is checked by the moisture of the air, 
and in such localities the snowy deposit accumulates every 
year on those mountains which pierce the frigid strata of the 
atmosphere. It might be expected, that in the course of a 
few thousand years, the height of such mountains would be 
considerably augmented by repeated atmospheric deposition 
of this character. This, however, is not the case. The piles 
of snow are soon taxed beyond their strength, crushed by 
their own weight and precipitated into the valleys below. 
Such is the origin of the avalanches which often occur in the 
Pyrenean and the Dofrafield Mountains, but which are most 
frequent in the Alps, and the hardy tenant of these bleak 
regions is exposed to perpetual dangers from the fall of these 
frail edifices, which atmospheric influences are continually rais- 
ing around him. 

Had snow and ice been as unyielding as the rocks on the 
earth's crust, such catastrophes would be averted for a longer 
time ; but when they occurred, they would be attended with the 
most awful consequences. If the constituents of granite were 
sublimed, like water, by solar heat, and deposited in a com- 
pact form on the tops of mountains, it would give them a 
continual augmentation of stature, until at last they must be 
crushed by the great weight of the pinnacles which they bore. 
In the present course of nature, atmospheric ^influences serve 
only to destroy mountain chains as they are raised by sub- 
terranean movements. But, in- whatever manner they may 
have originated, their height must have a limit depending on 
the strength of the materials which compose them ; and no 



12 THE INFLUENCE OF MAGNITUDE 

peak or table- lands could have an elevation of twenty miles 
above the level of the sea. If we could depend on the 
observations of some astronomers who announce the existence 
of so high a mountain on Venus, it may be regarded as 
an evidence that this planet can furnish much stronger mate- 
rials than we find in our terrestrial abode. 

It may be necessary to observe that the intensity of gravity 
is nearly equal on the surfaces of Venus and the earth, but it 
is -about twenty-seven times as powerful on the surface of the 
sun. If so great an attractive force prevailed on our planet, 
it would destroy all our lofty mountains and the great works 
of art ; while the larger animals would be crushed by the 
weight of their own bodies. Some phenomena exhibited by 
the sun have been ascribed to the presence of mountains 
several thousand feet high, and of valleys of corresponding 
magnitude ; but, to render the existence of such eminences and 
depressions possible, the solar matter must have many thousand 
times more passive strength than wrought iron ; and so extrav- 
agant a hypothesis cannot be admitted on the slender evidence 
which some astronomers adduce to support it. 

There is much greater scope for inequalities on the surfaces 
of small worlds. On the secondary planets the force of grav- 
ity is very small ; and it is so feeble on some of the asteroids, 
that a full grown man, translated to their surfaces, would weigh 
less than a pound. On such small celestial bodies, mountains 
might attain so great a size as to cause a considerable distortion 
from the planetary form peculiar to larger worlds ; and many 
facts show that the asteroids deviate very widely from exact 
spheres. . If the attractive power of the earth were so much re- 
duced, our mountain chains could attain a most gigantic size, the 
snows of unnumbered ages would be permitted to repose on 
their summits, and ice and snow might collect at the polar re- 
gions in so great a quantity as to lead to a change in the axis 
of rotation of our planet. Any of the asteroids which may 
contain a large proportion of air and water, would present 
very strange scenes to an inhabitant of the larger class of 
worlds. 

In investigating the conditions of stability, it has been usual 



ON STABILITY. 13 

for writers to regard solid bodies as absolutely unyielding in 
their character, while fluids are obedient to every impulse they 
receive. But, to obtain strict accuracy, it is necessary to con- 
sider how the cohesive strength of all material forms compares 
with the forces to which it is subjected, and how far it is in- 
fluenced by their magnitude. The drop of water ready to fall 
from a leaf has about the same relative strength and solidity 
as a ball of iron ten miles in diameter. Each of them would 
be crushed by its own weight if placed on an unyielding sur- 
face ; and even the latter would fall to pieces if it were raised 
from the ground by some superhuman power. Were our earth 
moulded into a cubical form, it would be soon crushed by the 
weight of its more prominent parts, until it acquired a shape 
differing little from that which it now possesses. Whenever 
matter is congregated into large masses, it pays little allegi- 
ance to the restraint of solidity, and complies with the condi- 
tions of equilibrium in fluid bodies. The existence of rivers of 
ice would not be credited if the movements of glaciers were not 
established by observation. 

The study of the conditions which give security to the works 
of art will serve to check some of the puerilities into which 
persons are liable to fall while speculating on the condition of 
other worlds. A little experience might have convinced the 
ancient astronomers of the impossibility of the existence of 
the large hollow spheres, in which they supposed that the 
stars and planets were fixed. Almost as extravagant a demand 
on the strength of materials was made in the seventeenth cen- 
tury by Riccioli, who regarded Saturn's ring as a mighty arch, 
supported by two gigantic pillars, which rested on the planet. 
More recently, astronomers have regarded these rings as two 
flat solid masses revolving around the planet, and having their 
weight equipoised by centrifugal force. It will be shown, 
however, that such an arrangement could only partially 
neutralize the weight, and that, under the most favorable con- 
ditions, they must be subjected to a strain about one hundred 
times as great as they could withstand if they were composed 
of wrought iron. All the knowledge which we can gain by ex- 
perience, however, discountenances the idea that the preser- 



14: INFLUENCE OF MAGNITUDE ON STABILITY. 

vation of so vast a mundane structure should be confided to 
the mere strength of the materials of which it is composed. 
When we observe the manner in which the works of art fall a 
victim to their inability to support their own weight, and how 
even the earth's crust has been rent by the convulsions of geo- 
logical times, we cannot but rejoice that the destiny of our 
world does not depend on the infirmities of its component parts. 
Whatever advantage the passive strength of materials may 
confer on man in the structure of his puny edifices, we may be 
assured that it has very little committed to its trust in the 
structure and arrangement of suns and worlds. 



THE DOCTRINE OF GRAVITATION. 15 



THE DOCTRINE OF GRAVITATION. 



Having shown that the stability of the works of art and 
the less gigantic works of nature depends on the passive 
strength of the solids composing them, I shall now proceed to 
the consideration of those properties from which matter derives 
its adaptation for purposes of a more sublime character. A 
general view of the leading properties or qualities of bodies 
may be requisite for a more extended survey of natural phe- 
nomena, but, as the present work is intended to treat only on a 
limited department of physical science, it will be necessary to 
confine our inquiries within a less extensive range. The pres- 
ent chapter shall accordingly be devoted to an examination of 
the mysterious force which takes the most prominent part in 
establishing the harmony of the material universe and giving 
a requisite degree of security to the several suns and world's 
it contains. 

While matter in itself cannot be said to have any predilection 
for rest or motion, nor any power to change its condition, it is 
associated with forces which, under certain circumstances, are 
ever ready to compensate for this physical inertia. The 
alliance between the material creation and the forces which 
maintain it in a state of activity is such that all bodies are 
capable of affecting the condition of others by exerting a pe- 
culiar influence, to which the term " attractions" has been ap- 
plied. Among these mysterious attractive influences, Gravi- 
tation holds the most important place, both in respect to the 
universality of its action and the greatness of the task which 
it is designed to perform. Cohesive attraction acts very en- 
ergetically at insensible distances, and gives materials the 
degree of strength from which man derives so many advan- 
tages. But it fails to afford the means for extending the size 



16 THE DOCTRINE 

of structures beyond a certain limit, and it would be utterly- 
inadequate to act the part which is assigned to Gravitation in 
the architecture of the heavens. 

Before contemplating the great achievements of this power 
in planetary movements, we must begin by tracing its opera- 
tion on our own sphere. It is the attraction of the earth that 
gives weight to all bodies around us, and causes them to de- 
scend when left unsupported. 

This mysterious influence proceeds from every portion of 
matter composing our globe; and in obedience to the forces 
operating in so many directions, bodies fall perpendicularly to 
the earth's surface. The attraction of mountains is manifested 
by the deflection of the plumb line, and that of smaller 
masses is revealed by experiments with the tortion balance ; 
x but even in bodies weighing a thousand tuns, it is too feeble to 
be recognized by ordinary observation. The powerful gravity 
of the earth renders us insensible to that exerted by such 
small bodies, in the same manner as the overpowering bril- 
liancy of the sun prevents us from seeing the stars during the 
day. In a region of space where no material action could 
arise from the presence of suns or planets, small masses 
should manifest their attractive influence, and if they had 
been previously at rest they should commence approaching 
one another with certain velocities. In such circumstances, 
two globes of lead, each containing one thousand tuns of the 
metal, and being separated by a distance of seven hundred 
feet, would be drawn together in the space of five days, and 
should acquire a relative velocity of about half an inch a sec- 
ond, previous to their collision. But if the earth were de- 
prived of its orbital motion, it would drop to the sun in the 
course of less than sixty-five days, and on arriving at his sur- 
face should have acquired a velocity about two thousand times 
as great as the most rapid cannon ball. 

The vast distances to which gravity extends its action, con- 
tributes to render it an effective agent in the government of 
our universe ; and its control is rendered more conducive to 
the harmony of nature by the impartial and uniform manner 
in which it acts on all matter. To lift one of the largest ships 



OF GRAVITATION. * 17 

without injuring its structure, it would be necessary to have the 
supports or the ties attached to its numerous parts, and to 
give a very proper adjustment of the force exerted on each of 
them. So, in like manner, the safety of each planet requires 
that the invisible chain which binds it to the sun, should lay 
hold of all its parts, and observe somewhat of a uniform char- 
acter in directing the movements of the whole planetary mass. 
Now this important object is attained by the gravity of the 
sun and the several other large spheres, for they manifest no 
partiality to the size or composition of bodies revolving around 
them, but under the same circumstances they affect the move- 
ments of all in a uniform manner. 

The first recognition of this important law must be ascribed 
to the celebrated Galileo. The experiments which he per- 
formed on the tower of Pisa proved, in opposition to the dog- 
mas of Aristotle, that small and large bodies fall to the ground 
from a given height in the same time. On the invention of the 
air-pump, the law was found to be more general than was first 
supposed, for in an exhausted receiver the lightest and the 
heaviest solids were observed to fall with almost the same ve- 
locity. The researches of Newton showed that it was not con- 
fined to the earth, for the matter of planets and comets, how- 
ever widely it may differ in density, paid the same allegiance 
to solar attraction. Experiments with the pendulum also prove 
the impartial action of terrestrial gravity. Pendulums of equal 
lengths, or having the same distance between the centers of 
motion and oscillation, vibrate in the same time, however the 
weight which they carry may differ in magnitude or composi- 
tion. The uniform manner in which gravity urges bodies 
in similar curves we may reasonably pronounce a conclusive 
evidence of the impartial effect it would exercise on falling 
masses of every description. 

If all bodies had the same composition, this uniform obedience 
to the force of gravity would not be surprising ; and, indeed, 
abstract reason might lead us to conclude that a tun and a pound 
of lead would fall to the earth or to any of the planets with 
equal velocities. But to the heterogeneous matter of which so 
many worlds are composed, the same unassisted reasoning could 
2 



18 THE DOCTRINE 

not apply; and, as all created objects differ so much in chemi- 
cal constitution and in external characters, we might naturally 
be led to suppose that they might manifest different relations to 
gravitative influence, some being unaffected by its power, or 
yielding to it with unequal degrees of velocity. Such a state 
of nature would not only deprive the material world of its pres- 
ent tranquility but would render it incapable of affording any 
intellectual treasures to the human mind. The sublime laws 
discovered by Kepler and Newton could then have no exist- 
ence ; and the researches of the mathematician and the astron- 
omer could be attended with no success in deciphering the phe- 
nomena of the heavens. 

The first experiments of Galileo, which made such an im. 
portant accession to the science of natural philosophy, were 
succeeded by attempts to determine the velocity which a body 
would acquire and the space it would describe in a given time 
when descending to the earth's surface. Direct observations 
were found inadequate to furnish results of sufficient accuracy, 
but Huygens showed that the time of falling through half the 
length of a pendulum might be found by dividing the time of 
its vibration through a small arc by 3.1416. From this it was 
calculated that, in the absence of the air, a falling body would 
move through about sixteen feet one inch in the first second, 
and it appeared from other investigations that the space it de- 
scribed would be proportional to the square of its time of de- 
scent. Thus was a definite measure obtained of the force of 
gravity at the earth's surface, and a foundation laid for our 
knowledge of the great power which plays so important a part 
in the control of the destinies of the Universe. 

The labors of Galileo and Huygens paved the way for the 
more profound discoveries of Sir Isaac Newton. Prior to his 
researches, astronomers had some faint impressions of the action 
of gravity between the celestial bodies, and many suspected its 
variation without being able to determine the law by which it 
was governed. It would be reasonable to suppose that, at great 
distances from the earth, terrestrial attraction must act over a 
wider range, and, supposing it to grow weak in proportion as it 
is expanded, it must be inversely proportional to the square of 



OF GRAVITATION. 19 

the distance from the center of our planet. That gravity, like 
light and other emanations, conforms to this law of diminution, 
was long the opinion of Newton, before he could find any 
means of verifying the conjecture. 

The lunar movements afforded the first opportunity of con- 
firming his anticipations and establishing his celebrated law. 
As sixty semi-diameters of the earth are nearly equal to the 
mean distance of the moon, the latter body should, according 
to the supposed law, feel terrestrial gravity about 3,600 times 
more feeble than it is at the earth's surface. It may be easily 
found that, in the absence of the air, a body would descend on 
the earth's equator 57,800 feet in one minute, and accordingly 
the deflection of the moon from the tangent of its orbit, in the 
course of a minute, must be one thirty-six-hundredth part of. 
this amount, or about sixteen feet. Now this agrees very closely 
with the actual deviation of the moon from a rectilinear course, 
an item which may be easily calculated from her motion and 
distance from the earth. Newton at first failed to obtain a co- 
incidence sufficiently satisfactory, as he used a very inaccurate 
estimate of the moon's distance and the earth's diameter; but, 
having subsequently obtained more correct values for these ele- 
ments, he was enabled to give a complete verification of his 
great law and to lay the foundation of Physical Astronomy. 

The idea that the force of gravity is exerted exclusively by 
the central matter of the earth was adopted by Huygens, and 
the direction to which falling bodies tend seemed for awhile to 
give it an appearance of plausibility. But subsequent investi- 
gations showed that the direction of superficial gravity is such, 
as might be expected if all the matter of the earth participated 
in producing it. Newton adopted the more reasonable hypoth- 
esis that gravitation is not confined to a certain class of bodies, 
but is the common property of all ; and he proposed, as a test, 
the effects of mountains in causing a deflection of the plumb 
line. The anticipated deflection was afterward found to occur 
at the Andes and Dofrafield Mountains, and also at Mount 
Schahalien, in Scotland. 

The observations made on the latter mountain led to a very 
important result. From its size and form it was calculated 



20 THE DOCTRINE 

that if it had the same density as the earth, it must cause a 
deflection of eleven seconds in the direction of the plumb line. 
The deflection, however, did not exceed five and a half seconds, 
and it was accordingly inferred that the rocks composing the 
mountain were only half as dense as the main portion of the 
terrestrial mass. By direct trials the specific gravity of these 
rocks and the earth with which they were associated amounted 
to two and three-quarters, and it was therefore concluded that 
our planet must have a density about five and a half times as 
great as that of water. From this estimate it may be easy to 
calculate that the whole terrestrial mass contains about 6000,- 
000000.000000.000000 of matter. This result is not materially 
different from that obtained by Cavendish, in his experiments 
with the tortion balance. It is, however, a tenth part less than 
the estimate obtained from the vibration of pendulums at the 
bottom and top of a deep mine, on a principle recently sug- 
gested by Professor Airy. 

Astronomers have also undertaken the task of weighing other 
worlds and ascertaining the condition of gravity at their sur- 
faces. Contrary to what might be first anticipated, it is only 
in the case of the larger celestial spheres that these efforts have 
been successful, and no determination has been hitherto made 
of the mass of the comets, the asteroids, nor even of the satel- 
lites of Saturn. It is easy to find at least an approximate com- 
parison between the amount of matter in worlds attended by 
satellites, especially if a case could be found in which two satel- 
lites described equal circles around their respective central 
bodies. In such circumstances the centrifugal force of each 
attendant must be proportional to the square of their velocities, 
and the attraction of the primaries to confine them in their or- 
bits must be in the same proportion. The first satellite of Ju- 
piter is situated at a distance of 278,000 miles from his center, 
and performs its revolution in 42 hours and 28 minutes. Had 
its distance been only 240,000 miles, we may find by Kepler's 
third law that it would perform its revolution in a little less 
than 34 hours, and thus move nearly 19 times as rapidly as 
our satellite does at the same distance. The centrifugal force 
would be, therefore, nearly 361 times as great as that of the 



OF GRAVITATION. 



21 



moon, from which it might be inferred that Jupiter must contain 
about 361 times as much more matter than our planet. I must 
observe, however, that, in consequence of the deviation of Ju- 
piter from an exact sphere, and other circumstances, the deter- 
mination of his mass by this means does not afford results as 
accurate as the astronomer requires, and the method of weigh- 
ing planets not attended by moons is so difficult that I shall not 
attempt to explain it in the present work. 

In consequence of the natural diminution of the force of 
gravity, the weight of the same body on the surfaces of differ- 
ent spheres is proportional to their mass divided by the square 
of their semi-diameters. Had the diameter of the earth been 
doubled, while its density remained the same, it would con- 
tain eight times its present compliment of matter, and attract 
the moon eight times as powerfully if the lunar orbit could re- 
tain its present dimensions. But bodies at its surface, being 
then eicrht thousand miles from the center, would have their 
weight only doubled, and not increased in an eight-fold ratio. 
The principal results of the investigations of astronomers on 
this subject are given in the following table. The first column 
gives the diameters of the sun and planets in English miles; 
the second is their estimated masses, that of the earth being 
regarded as unity ; the third, their densities in relation to that 
of water, and the fourth the weight of 100 pounds of terres- 
trial matter on their surfaces. 



Sun 882.000 

Mercury 2.950 

Venus 7.800 

Earth 7.812 

Mars 4.100 

Jupiter 88.640 

Saturn 75.000 

Uranus 34.600 

Neptune 38.000 



359.551.000 


1.37 


074 


6.71 


885 


5.11 


1.000 


5.44 


132 


5.21 


338.475 


1.32 


101.066 


0.76 


14.255 


0.97 


24.880 


1.25 



2858 
46 
92 

100 
54 

262 

112 
75 

109 



The numbers in the fourth column cannot have any pre- 
tensions to very great accuracy, as the force of gravity is not 
the same at all parts of the surfaces of the planets, and bodies 
have a less weight at their equatorial regions than at their 
poles. On our own planet the greatest difference of weight 
arising from geographical position, is little more than \ per cent; 



22 THE DOCTRINE 

but on the surface of Jupiter it is about thirty times as great. 
It is now almost two centuries since the influence of latitude 
on the weight of bodies was first recognized by scientific men. 
It had been long remarked that clocks which kept correct time 
in places in Europe were too slow when carried nearer to the 
equator, a fact evidently indicative of the weakness of gravity. 
Sir Isaac Newton and Huygens, about the same time, ascribed 
these phenomena to the earth's rotation, and the former con- 
cluded that the same cause would give our planet a spheroidal 
form. This idea was countenanced and probably first sug- 
gested by the observations of Cassini, who showed that the 
planet Jupiter deviates considerably from an exact sphere. 
From subsequent measurements of degrees in different lati- 
tudes, combined with experiments by means of a pendulum, it 
appears that the difference between the equatorial and polar 
diameters is about twenty-six miles. 

The centrifugal force attending circular motion arises from 
the natural tendency of matter to take a straight course, and 
varies according to a law first pointed out by Huygens. It is 
proportional to the weight of the body multiplied by the square 
of its velocity, and divided by the radius of the circle in which 
it moved. If the velocity were half what the body would ac- 
quire by descending freely through a space equal to the 
diameter of the circle it described, it would have a centrifugal 
force capable of balancing its weight. This, for instance, 
would be the case with the circumference of a wheel sixteen 
feet in diameter, and making seven revolutions in the course 
of twenty-two seconds. A velocity one hundred times as 
great would rend such a wheel into fragments, though it were 
formed of the strongest materials which the artisan can com- 
mand. 

The equatorial gravity of the earth is reduced one-third per 
cent, by centrifugal force; and a farther reduction arises from the 
spheroidal form of our planet. It is commonly stated, that a 
rotation seventeen times as rapid as it is at present would be 
required to make bodies lose their weight at the equator ; but 
this has been determined on a supposition that our terrestrial 



OF GRAVITATION. 23 

fabric could preserve its form unchanged, or that its materials 
were capable of sustaining, by mere strength, the weight of a 
column of matter four thousand miles high, which presses 
down from the polar regions. Supposing the earth to be 
homogeneous, it may be shown that a rotation ten times as 
rapid as that which it now possesses would be inconsistent 
with its stability. In consequence of the low density of Jupiter 
and Saturn, a much less rapid rotation would be fatal to their 
planetary existence. 

In the present work it will not be possible to give an 
adequate idea of the operation of gravity in maintaining so 
many orbs in their harmonious condition, or to do justice to 
the merits of the discovery which has revealed its existence. 
Every new development in astronomical science gives fresh 
illustrations of the vast extent of its controlling influence, and 
of the universality of the law to which it conforms. The most 
decisive evidences of the truth of this important law, are de- 
rived from an investigation of the movements of the several 
attendants of the sun and the large planets. But, as such a 
course of inquiry is attended with great difficulty, and as it is 
farther removed from the pale of ordinary experience, I shall 
commence by tracing the work of this mysterious power in the 
phenomena which occur around us, and to which attention is 
most likely to be directed. 



24 THEORY OF 



THEORY OF THE TIDES. 



While the solid crust of the earth preserves the degree of 
stability which man's existence demands, the liquid and gas- 
eous elements which invest it are obedient to the impulses 
which they receive from agencies of incessant activity. The 
freedom of movement which water has acquired gives it an 
adaptation for serving great ends, and the vast ocean which 
covers nearly three fourths of the earth's surface> exhibits 
changes on a scale commensurate with its magnitude. So vast 
an amount of restless matter stationed on our earth may seem 
calculated to inspire us with some degree of apprehension for 
the safety of man's abode. But the mighty realm of waters 
pays allegiance to the terrestrial gravity which confines them to 
the lower parts of the earth and prevents their invasion of the 
land. The violence of storms can only affect, on a very lim- 
ited scale, the level of the ocean, and a host of streams and 
rivers strive in vain to give it a permanent elevation. But a 
periodical rise and fall takes place in obedience to a power, 
which, though eight thousand leagues distant, is permitted to 
act a part in the affairs of our planet. 

This great tidal movement occurs twiee in a little less than 
twenty-five hours, and consists of a general swelling and sink- 
ing of the great watery domain ; so that, in many places, there 
is a difference of several feet in the height of the water at dif- 
ferent times of the day. To produce such a swell over an ex- 
panse of waters many millions of miles in extent, may be justly 
regarded as involving an extraordinary manifestation of power, 
yet it is in extensive oceans and not in lakes or inland seas that 
this great movement is exhibited. The sublime phenomenon 
is not to be witnessed in the Baltic, the Black, or the Caspian 
Seas, and it is only in a small part of the Mediterranean that 
it can be perceived even on an insignificant scale. The high 



THE TIDES. 25 

tides of the Indian Ocean were accordingly a source of surprise 
to the soldiers of Alexander, and those of the Atlantic pre- 
sented to the Roman legions a similar contrast to the station- 
ary character of the waters on their own coasts. What 
caused these great pulsations of the deep could not be discov- 
ered at a period when physical science was in so imperfect a 
condition ; but it was observed, even as early as the times of 
Pliny, that the mean interval between two tides is equal to the 
average length of a lunar day. The evident relation between 
lunar and oceanic movements was accounted for by Des- 
cartes, on the supposition that the moon exerted a pressure on 
terrestrial matter ; but Kepler was the first to ascribe it to the 
moon's attraction. The precise manner in which this power 
operates was first pointed out by Newton, and the investiga- 
tions of Laplace and Airy have done much to improve the 
theory by showing that this tidal force produces the observed 
elevation of the waters in different localities. 

In the infancy of modern astronomy, it was supposed that 
the motion which Copernicus ascribed to the earth, must be 
fatal to the repose of all bodies on its surface. But as soon as 
a more correct knowledge of the properties of matter began to 
prevail, it was perceived that our globe could endure a much 
greater velocity without sustaining any injury or disturbance, if 
all its parts partook of the movement, and no obstacle occurred 
to check its rapid career. Even the greatest external force that 
might operate on a planetary body, could not affect the stability of 
its lands or seas, provided it impelled the whole mass with the 
same velocity and in the same direction. The attraction of 
the sun, on his surrounding worlds, very nearly fulfills these 
conditions, as it acts on bodies without any regard to their 
composition, and it bends all parts, every portion of our terra- 
queous globe to almost the same extent from their rectilinear 
course. Eight thousand miles is so small in comparison to 
the sun's distance, that the force and direction of his attraction 
throughout the extent of our planet is comparatively trifling, 
yet it is large enough to affect the level of our oceans, and to 
produce solar tides. 

But although the moon exerts a much less amount of abso- 



^0 THEORY OF 

lute force on our planet, her proximity gives her a greater 
control over the movements of our waters. The manner in 
which this effect is produced I shall attempt to explain. Be- 
coming weak, as the square of the distance increases, her at- 
traction on different portions of the earth are subject to a varia- 
tion of more than six per cent., and she draws the earth's 
center with a force which may be considered as an average of 
what she exerts on the whole terrestrial mass. Taking this 
as its par value, it will readily appear that her attractive power 
is about three per cent, above par at the nearest part of the 
earth, and three per cent, below par at the place most re- 
mote. From the laws of inertia, it is evident that the bond 
subsisting between our planet and its oceans is weakened in 
the same degree — whether the waters are attracted more or 
less than the whole terrestrial fabric — and accordingly about 
three per cent, of the lunar attraction is expended in diminish- 
ing their weight on the two opposite parts of the earth. To 
these two localities the waters are urged from the regions 
around in consequence of a slight propulsive power which the 
moon exerts when she acts obliquely to the earth's surface. 
But while the moon diminishes gravity on the two opposite 
parts of our planet, she increases it in the zone lying interme- 
diate between them, and comprising over one half of the 
earth's surface. The last effect arises from want of parallel- 
ism in the direction of lunar gravitation, and it assists in pro- 
ducing the tidal movement. The waters of our oceans, ever 
ready to accommodate themselves to the changeable condition 
of terrestrial gravity, are compelled to rise and fall period- 
ically, as different portions of our globe are turned to the lunar 
orb. 

It may seem necessary to take into consideration the centri- 
fugal force arising from the earth's revolution around the sun, 
and also around the center of gravity between itself and the 
moon. This, however, is not the case. The component parts of 
a planet could not be disturbed in any degree by moving through 
space in a straight line, however great may be its velocity, and 
the attraction of other bodies is the only cause that prevents 
celestial motion from being rectilinear. It is therefore evident 



THE TIDES. 27 

that the estimate of the effects arising from the attraction of 
the sun and moon on the earth, includes all the disturbances 
attending its curvilinear motion. The rotation around its axis 
only serves to give a permanent elevation to the matter of the 
equatorial regions ; and I may caution the reader against the 
views of some popular writers, who represent the earth's cen- 
trifugal force as the cause of the tides on the side of our globe 
most remote from the moon. How far the diurnal velocity of 
different parts of the earth modifies the effects of the tidal 
force by limiting the time necessary for the oscillations of the 
ocean, will be presently considered. 

Were the moon placed at half her present distance from us, 
her attraction on the earth would be four times as great as it is 
at the present time, but a double per centage of it would be 
then engaged in disturbing our seas, and the tidal force would 
be therefore increased in an eight-fold proportion. From more 
abstruse investigations, it appears that the influence any body 
exerts in producing tides is directly proportional to its mass, and 
inversely as the cube of its distance. There is a very marked 
difference between the heights to which the waters are elevated 
by the moon when in her greatest and least distances from the 
earth, but a more decided influence must be ascribed to her 
declination and her position with respect to the sun. 

The mass of the solar orb is about 29,000,000 times as great 
as that of the moon, and his attractive force on the earth is over 
180 times as powerful, but this varies so very slightly that his 
tidal influence is small, being only two-fifths of what arises from 
the action of our satellite. When both luminaries are in con- 
junction or in opposition they cause the watery domain to swell 
in the same direction, and by their united action the tides attain 
an unusually great height. When the moon is in her quadra- 
tures, or in direction perpendicular to that of the sun, the at- 
traction of both bodies is exerted in concentrating the water on 
four different localities, and a low tide is obviously the conse- 
quence. 

But it must be remembered that effects of this kind are always 
manifested subsequent to the operation of the causes from which 
they arise. Our days reach their highest temperature a few 



28 THEORY OF 

hours after the sun has left the meridian, and the heat of sum- 
mer is greatest several weeks after the 21st of June, when the 
sun attains his greatest declination. In like manner the waters 
of the ocean do not swell to their full height until some hours 
after the moon has passed the meridian, and the spring tides 
are not generally manifested on their grandest scale until about 
two days after new or full moon. 

From the great irregularities in the bed of the ocean and 
other causes, it is impossible to attain any great precision in 
computing the height to which its waters swell in different local- 
ities. According to the equilibrium theory of Sir Isaac New- 
ton, the increased height in the several places should correspond 
to the reduction of gravity by solar and lunar action. If the 
earth's diurnal motion were so slow that several days should 
elapse between two consecutive tides, we may reasonably ex- 
pect that this theory would be successful in indicating their 
height in almost every part of the ocean, but at present it only 
accords imperfectly with the results of observation. Laplace 
has endeavored to grapple with the difficulty by estimating the 
influence of the lunar disturbance in imparting motion to the 
waters ; but the wave theory of Professor Airy may be consid- 
ered as furnishing more satisfactory results, and can be more 
readily presented in a form intelligible to the general reader. 

As the waters, on attaining their full height by lunar attrac- 
tion, begin to sink, they must contribute by their pressure to 
swell the part of the ocean immediately west of them, thus 
causing a tide-wave to follow the moon. If the depth of the 
sea were sufficiently great, such a tide-wave would travel with 
a velocity of several hundred miles an hour, while the water 
did not change its actual position more than a few feet. Ac- 
cording to the estimate of Professor Airy, a depth of about four 
miles would be sufficient to permit a tide-wave, 7,000 miles 
wide, to more at the rate of 600 miles an hour. If a canal so 
deep occupied the region between the parallels of 48 and 52 
degrees, and extended quite around the earth, the tide- wave 
formed in it would keep pace with the moon's movements. 
Now the swellings of such a wave, like the vibrations of a pen- 
dulum, are always increased by the application of a force which 



THE TIDES. 29 

acts in unison with their movements ; and accordingly, if the 
supposed channel were suddenly brought into existence, the 
tide, commencing on it on a small scale, would be augmented 
by every return of the moon, until at last it acquired a gigantic 
magnitude. 

As the distance from the equator increases, the great wave 
requires less velocity to keep pace with the lunar body, and ac- 
cordingly, in high latitudes, our satellite is enabled to make a 
more decided change in the level of the waters, though pos- 
sessed of an inferior tidal force. In no place is there a better 
opportunity afforded for the watery manoeuvre than in the 
Southern Ocean, and the high tides in this region have their 
influence felt in very distant localities. The tides are generally 
small in the tropical seas, as the depth is not great enough to 
allow the wave to travel with the requisite velocity; and the 
swells on the Indian Ocean are to be ascribed in a great meas- 
ure to the movements of the waters around the south pole. 
Indeed, Whewell goes so far as to maintain that the Atlantic 
tides are principally due to a derivative wave transmitted from 
the Antarctic Ocean; but Airy questions this conclusion, and 
maintains that the effects of the southern tides on the level of 
seas so remote must be extremely small. 

From the experiments of Mr. Russell it appears that the tide- 
wave, on meeting a coast, experiences a kind of reflection, and 
this serves to make the waters rise to an extraordinary height 
in some localities. To circumstances of this character we may 
ascribe the great height of the tides in the Bay of Fundv, as 
well as in many harbors of the British Isles and the Atlantic 
coast of France. But if the two waves, instead of swelling 

o 

simultaneously at the same locality, exert their tidal influence 
in succession, their effect will be only manifested on a more in- 
significant scale ; and this accounts for the great difference in 
the rise of the waters in places which are separated by an in- 
considerable distance. 

The disturbing force of the moon, which gives rise to the 
great oceanic movements in the manner described, is nearly 
equal to one seven-millionth part of terrestrial gravity, and it 
is scarcely capable of causing a reduction of a single grain in 



30 THEORY OF THE TIDES. 

the weight of one thousand pounds of water. But the pressure 
of the fluid in distant parts of the ocean is indispensable to the 
phenomenon, and in seas which have no adequate connection 
with the ocean no tides can take place. In a sea two thousand 
miles long the moon may give such a change to the weight of 
the waters at both ends as to produce a slight oscillation, and 
to an action of this kind we may ascribe the slight tides which 
can be barely recognized in some parts of the Mediterranean. 



EFFECTS OF THE TIDES. 31 



EFFECTS OF THE TIDES. 



From the manner in which the moon affects the condition of 
our ocean many have endeavored to find indications of her in- 
fluence on our atmosphere, and also on the vast body of lava, 
which is supposed to occupy the interior of our planet. It has 
been supposed that if the subterranean regions contained such 
an ocean of liquefied rock, it must experience semi-diurnal tides, 
and produce regular volcanic eruptions, in consequence of the 
moon's disturbing action. But it would seem that in these con- 
clusions due weight has not been given to the restraints im- 
posed on the movements of the lava by its own imperfect fluid- 
ity and the resistance of the earth's crust. It may, however, 
be interesting to refer to another operation, which contributes 
in a more indirect manner to make the more solid kinds of ter- 
restrial matter yield to the feeble influence of lunar power. 

The great rivers of our continents and islands descend very 
little in the latter part of their course, and indeed friction is so 
trifling an impediment to the movement of the vast bodies of 
water they roll to the ocean, that a very slight declivity is suffi- 
cient to impart to them a considerable velocity. Now the rise 
of a high tide checks or frequently reverses the motion of their 
waters, and imposes on them the necessity of discharging an 
excessive quantity while the ocean is returning to its lowest 
level. In such circumstances the eroding and transporting 
power of a river is so much increased that it is capable not only 
of clearing its mouth of sediment, but also of giving it a con- 
siderable expansion in the course of many years. For this 
reason the mouths of rivers which flow into seas having high 
tides are always remarkable for the great breadth of their 
mouths, while those terminating in lakes or tideless seas are 
characterized by an opposite peculiarity. In the latter case, 



32 EFFECTS OF 

the motion of their waters and their transporting power is 
nearly all lost on reaching the sea, and the sediment which they 
convey is expended in forming deltas at their mouths. Ac- 
cordingly, deltas are to be found at the terminations of those 
rivers of Europe which empty into the Mediterranean Sea, 
while those flowing into the Atlantic Ocean show a great en- 
largement of their channels in the last part of their course. 
The deltas and the narrow mouths of the Nile, the Danube 
and the Wolga exhibit a striking contrast to the wide estuaries 
by which the rivers of the British Isles communicate with the 
Atlantic. In the New World we find a repetition of the con- 
trast, on comparing the rivers which flow into the Gulf of Mex- 
ico, where the tides are insignificant, with those of Maine and 
New Brunswick. On the coasts of the latter States, the eleva- 
tion of the tides is so considerable that all obstructions are re- 
moved from the mouths of their rivers and streams, and they 
accordingly possess much better harbors than can be found on 
the coasts of the Southern States. 

In the latter case, however, another cause is concerned in 
producing the effects under consideration.' The rivers of 
Maine and New Brunswick all pass through numerous lakes, 
and this deprives them of so much sediment as to render them 
incapable of forming large deposits on reaching the ocean. In 
the course of numerous ages, when these lakes will have been 
drained or silted up, and when their rivers introduce more 
solid matter into the sea, their estuaries may be expected to 
diminish in size, and some of them may entirely disappear. 
The Ganges is so highly charged with silt that it has formed 
a large delta at its mouth, notwithstanding the opposition it 
must have received from a tide of considerable height. 

As a general rule, the action of high tides is attended with a 
considerable waste of the land. Their devastations are ex- 
hibited in a very marked degree on the coasts of France and 
England, and in many parts of the latter the encroachment of 
the seas, according to Lyell, amounts to twelve feet annually. 
Several islands in the neighborino- seas have fallen victims to 
its destroying influence within the memory of man, and Heligo- 
land has only a very small portion of the area it possessed 



THE TIDES. 33 

about a thousand years ago. But it would seem that these 
destructive effects are to some extent compensated by the de- 
posits which take place in the shallow parts of the sea, es- 
pecially in the shallow water where the tide wave acts with 
diminished force. Lieutenant Davis shows that an important 
geological action must be ascribed to the tides, and that they 
are making a considerable accession to the land on many parts 
of the coast of the United States. 

Their effect, however, is somewhat modified by the presence 
of very large rivers. The great quantity of sediment which 
these transport, instead of forming deltas at their mouths, is 
swept out to a considerable distance from the shore and deliv- 
ered to marine currents, which scatter over a very extensive 
surface. In this manner we may account, not only for the 
origin of the sedimentary rocks composing the earth's crust, 
but also for the regular stratification which they exhibit, and the 
vast distance to which many of the strata extend. Indeed, 
the structure of these rocks is generally so different from what 
we might expect in tideless seas, that we may regard it as an 
indication that, at the remote geological period which they 
represent, the moon rolled around our planet and com- 
pelled the waters to pay homage to her attractive power. 

But the moon is not to be regarded as an agent concerned 
in the works of nature alone. Of the various sources of 
power placed at man's disposal, the tides hold an important 
position, and, though at present little used for the purposes of 
art, they may be found very useful in future times when sup- 
plies of coal are exhausted, and the reign of steam must de- 
cline. Lesley estimates the mechanical effect of the tides as 
equivalent to two thousand five hundred times the labor of the 
human population, but it must be remembered that with man's 
limited resources, only a small portion of this can be rendered 
available for the execution of his designs. 

It is regarded as an axiom in natural philosophy, that no 
power can be created, and, accordingly, it may be inferred 
that either the lunar or terrestrial motion must experience a 
permanent change, corresponding to the mechanical effect 
which the tides are capable of producing. Without the earth's 
3 



34 EFFECTS OF 

rotation, these movements would occur only once in about two 
weeks, and be attended with much less decided consequences 
than they now manifest on our coasts, so that it may be pre- 
sumed that the principal loss falls. on the momentum of our 
terrestrial globe. Laplace, however, has been led to conclude 
from his researches, that the fluctuations of our seas are not 
attended with any loss of the earth's diurnal motion, and from 
the eclipse observed in the time of Hipparchus, he calculates 
that the diminution in the length of the day, has not amounted 
to one-hundredth part of a second during the last two thou- 
sand years. In this estimate, the movements of the moon 
have been taken as a criterion forjudging the uniformity of 
the earth's rotation, but I shall endeavor to show that the stand- 
ard of comparison cannot be relied upon ; the velocity of the 
lunar orb being affected by the swelling of our seas. As this 
investigation is attended with much difficulty, I shall begin 
with cases more simple than those presented in nature, and 
show how a satellite feels the reaction of very great disturb- 
ances which it may occasion on the oceans of a primary planet, 
when the tidal movement is exhibited on a most gigantic scale. 

Although the moon's disturbing power has been considered 
equal on opposite sides of the earth, it is in reality between 
four and five per cent, greater on the nearest part than it is on 
the most distant. If an ocean of uniform depth covered our en- 
tire globe, it would swell a few inches higher eastward of the 
moon's place than it did in the opposite locality. Had our 
moon been only sixteen thousand miles distant, she would be 
followed by a tide -wave twice as high as that preceding her, 
if the waters of our planet were sufficiently copious to afford 
full scope to her action. If the lunar orbit were reduced to 
smaller dimensions, the disproportion between the two tide- 
waves would be augmented. 

A body describing a circular orbit, four thousand miles 
above the earth's equator, would complete its revolution in a little 
more than four hours. If an iron globe, three hundred miles 
in diameter, revolved in such an orbit, while a deep ocean oc- 
cupied the tropical regions, the tide-wave in advance of this 
body would be very insignificant in comparison to that follow- 



THE TIDES. 35 

Ins: it at the rate of five thousand miles an hour. Now, in con- 
sequence of the attraction of the great body of waters swelling 
behind it, the supposed satellite must be drawn, not directly to 
the earth's center, but always to a point a little westward of it, 
and a constant loss of motion must be an inevitable consequence. 
If the tides failed to rise, this loss would not occur, for the force 
of gravity would be always directed to the center of our planet; or 
if its direction were changed by the presence of a mountain, the 
deflection would be alternately to the east and west, so that 
the motion would be accelerated and retarded without receiv- 
ing any permanent alteration. The reaction on a satellite from 
the swelling of the seas cannot be considered analogous to 
planetary disturbances; for, in consequence of the change in 
the relative positions of the several planets, their disturbing 
influence assumes a periodical character, and the loss and gain 
of motion is evenly balanced after a series of revolutions. 

Had the moon's orbit been a circle situated in the plane of 
the equator, and only forty-eight thousand miles in diameter, 
her revolution would be performed in a little less than twenty- 
one hours ; and she would be followed by a mighty tide-wave, 
moving from west to east, at the rate of about one hundred and 
sixty miles an hour, in the tropical seas, while a wave of much 
less magnitude traveled on the opposite side of the earth with 
an equal velocity. The unequal attraction exerted by these 
two bodies of water would always give a westward deflection 
to the force which retained the moon in her orbit, and con- 
tinually destroy an imperceptible part of her motion. But, at 
the same time, the action of the moon on the waters which 
form these two tide-waves would give them a slight translatory 
movement, and thus accelerate the rotation of the earth. 

It must be observed, however, that the moon, by sustaining 
a partial loss of its motion, would be consigned to a smaller 
orbit, exposed to a more powerful terrestrial attraction, and 
compelled to move with increased rapidity. However para- 
doxical it may seem to the general reader, it is well known to 
those acquainted with physical astronomy, that the result of a 
resisting medium in destroying the motions of planets or comets 
would only be to make them perform their revolutions around 



36 EFFECTS OF 

the sun in a more brief period. It appears, therefore, that if 
a primary planet had a satellite revolving around it in a period 
less than one of its days, and producing tides in its oceans, the 
rotation of the first body and the revolution of the second must 
be continually accelerated, while the distance between them 
underwent a corresponding diminution. These changes would 
take place so very slowly that a long period might elapse before 
they were detected, and indications of their occurrence could 
be obtained only by indirect evidence derived from the manner 
in which they served to account for observed phenomena. 

The result of tidal action is somewhat different when the 
revolution of the satellite occupies more time than the rotation 
of a prim ary'sph ere. In the present condition of our own planet, 
for instance, the tide-wave, which has the greatest disturbing 
influence, is always east of the moon, and must add to her 
velocity while it retards the earth's diurnal motion. JSTow the 
additional velocity imparted to our satellite only serves to en- 
large her orbit and increase the time of her revolution, so that 
there is a diminution both of lunar and terrestrial motion, de- 
pending on the difference of the attractive forces of the waters 
on opposite sides of the earth. From the nature of the ques- 
tion it seems extremely difficult, if not absolutely impossible, 
to obtain a precise estimate of the loss of momenta which the 
two mighty bodies sustain. In addition to the complicated 
character of tidal movements, some allowance is to be made for 
the effects of a resisting medium on the motion of the moon 
and earth, and, perhaps, for the influence of secular refrigera- 
tion in accelerating the rotation of the latter body. It would 
seem, however, that the length of the day during the last two 
thousand years cannot far exceed the limits prescribed by 
Laplace, although we cannot recognize the propriety of re- 
garding lunar movements as an exact standard of comparison 
for those of the earth. 

The form of a celestial body might afford some faint indica- 
tions of the slow diminution of its rotary motion during the 
lengthy ages of past time. The planet Mars has a flattening 
about ten times as great as could be expected from its present 
rotation, and this would lead us to suspect that his diurnal 



THE TIDES. 37 

motion and centrifugal force must have diminished very much 
since he was molded into his present shape. Yet there seems 
to be no cause to alter this motion except the resistance of the 
rare fluid disseminated through space. But the earth being 
much larger than Mars, and possessed of greater attractive 
power, its crust would be incapable of restraining the tendency 
of the lava to seek a new equilibrium on the decline of its cen- 
trifugal force. In a former publication, I have shown several 
reasons for believing that the last great geological revolution 
consisted in a sudden upheaval of the polar regions, as the re- 
duced condition of the earth's rotation called for a nearer ap- 
proximation to a spherical form. 

If the loss of lunar and terrestrial motion were commen- 
surate with the mechanical action of the tides, a lapse of many 
centuries must be required to render it appreciable, for the 
amount of force associated with the swelling of the ocean is 
very insignificant in comparison to the momenta of such stu- 
pendous spheres. Indeed, it is extremely difficult for us, from 
our experience with the powers around us, to form any just 
conception of the vast amount of force embodied in mighty 
worlds moving with such amazing rapidity. Archimedes wished 
for a means of applying his machinery to the movement of 
the earth. Had his wish been complied with, it would only have 
served to reveal the weakness of the powers he could command, 
and their inadequacy to the task which he attempted to achieve. 
A thousand gigantic machines, each capable of performing one 
hundred times as much work as can be accomplished by the 
labor of the whole human population, would require over a 
million of years to impart to the earth its present diurnal mo- 
tion. Yet this diurnal movement gives our globe only a very 
small momentum, compared with that arising from the im- 
mense velocity with which it describes its annual orbit, and 
such mighty globes as Jupiter, Saturn and the sun exhibit a 
far greater embodiment of power in traveling through space. 
It appears, however, that the majestic movements of the plan- 
ets, besides attaining many other important ends, may occa- 
sionally serve, through the medium of tidal action, to place a 
vast fund of power at the disposal of their inhabitants. 



38 EXCESSIVE TIDAL ACTION 



CASES OF EXCESSIVE TIDAL ACTION AND PLANETAKY 
INSTABILITY. 



In studying the manner in which the moon produces such 
decided movements of our seas, we may be Jed to gloomy 
anticipations in regard to the condition of many worlds, which, 
with a very inferior force of gravity, are exposed to far more 
violent disturbances from foreign bodies. Such is the fate of 
many of the secondary planets which are separated only by 
comparatively small distances from primary spheres of great 
magnitude. When we consider that Saturn has nine thousand 
times and Jupiter thirty thousand times the amount of matter 
contained in our moon, we may be disposed to regard the at- 
traction of these great orbs as fatal to the repose of the lesser 
worlds which attend them. If our globe, without having its 
diurnal motion altered, could exchange orbits with Jupiter's 
first satellite, our tidal force would become about nineteen 
thousand times as great as it is at the present time, and it 
would increase to fifty-six thousand times its present energy, if 
we were transferred to the orbit of the nearest satellite of Sat- 
urn. Under such circumstances, our lands and even our 
mountains w r ould be covered by the regular swelling of our 
oceans, nor could even the solid crust of the earth withstand 
so violent a disturbance. More serious consequences may be 
expected on these second rate planets, which, from their low 
density and feeble attractive power, are less capable of hold- 
ing their parts together, and keeping commotions under proper 
restraint. Even on the most distant satellites of Jupiter, a con- 
tinual succession of deluges may be expected, if extensive 
oceans covered their surfaces, and were permitted to fluctuate 
in obedience to the impulse of the enormous tidal action to 
which they are exposed. 

These disastrous consequences which must consign so large 



AND PLANETARY INSTABILITY. 39 

a number of the occupants of space to a state of comparative 
inutility, could only be prevented by a peculiar arrangement, 
which, though observed only in a few cases, is supposed to 
characterize the movements of all the secondary planets. It 
is well known that our moon's rotation keeps pace with its 
revolution, so that the same side is always turned to us ; and 
from some appearances in the satellites of Jupiter and Saturn, 
we have reason to believe that there is a similar equality of 
their orbital and diurnal movements, and that they always 
present the same side to the central body. Accordingly, 
though the powerful attraction of Jupiter may cause the liquid 
and even the solid parts of his nearest moon to yield to its in- 
fluence, it w T ould keep them continually elevated in the same 
regions, and the small world would thus acquire a permanent 
deviation from a perfect sphere. Had its orbit been an exact 
circle, and coincident with the plane of its equator, the level and 
the boundaries of its lands and seas would not be changed by the 
action of the primary. If the orbit were elliptical or much in- 
clined to the plane of rotation, the attraction of the central 
mass would not be capable of keeping an equal amount of 
water at the same elevation at all times, and there would be a 
tidal movement at every revolution. The orbits of Jupiter's 
satellite, however, deviate little from true circles, and there 
is reason also to expect that they have the other condition 
necessary to save them from the effects of destructive tides, 
whatever be the relation subsisting between their lands and 
seas. 

Although this arrangement is calculated to give secondary 
planets the greatest degree of repose and security which it is 
possible for them to obtain, it cannot be considered as furnish- 
ing a guarantee for their stability under all circumstances. This 
will appear evident on a further extension of the present inves- 
tigation. The first satellite of Jupiter, which is separated from 
him by a distance of 278,500 miles, performs its revolution in 
the space of 42 hours and 28 minutes. In conformity with 
the rule just noticed it must revolve around its axis once in the 
same time, and the centrifugal force arising from this rotation 
must diminish its equatorial gravity by the one hundred and 



40 EXCESSIVE TIDAL ACTION 

eighty- fourth part of its force. The attraction of the primary 
would produce double this reduction of gravity at the points in 
conjunction and in opposition with him, and give it an increase 
of over one-half per cent, in places ninety degrees distant. 
Accordingly, there would be at least a difference of about two 
and one-fifth per cent, between the weight of the same body at 
the poles of the satellite and the parts nearest and most dis- 
tant from Jupiter. The change of form which the small planet 
would necessarily experience in such circumstances, would 
make a still greater difference in its superficial gravity. 

The first satellite of Saturn is situated at a mean distance 
of 118,000 miles from his center. Had the first attendant of 
Jupiter been confined to so small an orbit, it would perform its 
rotation and revolution in less than twelve hours, and there 
would be a difference of twenty-seven per cent, between the 
greatest and least force of gravity at its surface, even if no de- 
viation from a sphere occurred. But the ellipsoidal form, which 
it could not fail to assume in such circumstances, would make 
the difference so considerable that the same body would have 
double as much weight at the poles as at the points nearest to, 
and most distant from, the central orb. Gravity would be 
completely neutralized at these localities had the satellite been 
confined to an orbit of only 160,000 miles in diameter; and 
even the matter which extends to the center of the body would 
feel to nearly the same extent the effects of the disturbance 
of Jupiter in depriving it of weight. The force of gravity 
being thus suppressed in two directions, it would be impossi- 
ble to maintain the planetary form. As no known materials 
would be capable of supporting by mere passive strength a 
structure over one thousand miles high, the want of a proper 
balance between its parts would render the dismemberment 
of the satellite inevitable in such a close proximity to the cen- 
tral body. 

To determine how far the earth's diurnal motion should be 
augmented to destro}^ equatorial gravity and reduce it to an 
unstable state of equilibrium, has been long an object of math- 
ematical investigation. It must, however, be more interesting 
to know how far the orbit of a satellite should be reduced be- 



AND PLANETARY INSTABILITY. 41 

fore its own rapid motion and the disturbance of the primary- 
would reduce it to a similar condition of instability. Were both 
bodies equally dense and the disproportion between their size 
as great as is generally found in secondary systems, the satel- 
lite could not revolve in security in an orbit having a diameter 
less than five times the radius of the primary. But, although 
it is extremely difficult to attain great precision in calculating 
the size of the smallest orbits which it might be possible for 
the celestial bodies to describe, it is evident that the resistance 
of a space-pervading medium, after innumerable ages, would 
bring a satellite into such a proximity with the central body as 
to lead to its dismemberment by the disturbing power of the 
latter. If satellites were originally placed in such narrow or- 
bits, the consequences would not be materially different. 

On the dilapidation of the planetary structure by the causes 
alluded to, the resulting fragments would start forth in such 
paths as their motions would assign to them. From mutual dis- 
turbances and the deviation of the central body from a perfect 
sphere, the numerous ellipses which the fragmentary host 
would describe would rapidly change the position of their trans- 
verse axes to an extent depending almost exclusively on their 
size and their eccentricity. The fragments, which pursue 
nearly the same path, being therefore equally affected by this 
change of position, or apsidal movement, as it is called, will 
follow one another during several revolutions, forming a line 
which remains long unbroken. The several rings of fragments, 
being thus compelled to roll one within another, must be p-rad- 
ually brought into a circular form. It may be easily shown 
that there is the least violence from collisions when the orbits 
are true circles, and that the amount of friction between them 
is a minimum when they obtain a uniform distribution around 
the central planet. The ultimate transformation of the mighty 
wreck into a uniform girdle around the primary, may be natu- 
rally expected from the obvious tendency of the fragments to 
seek the nearest approximation to a state of absolute repose. 

Of the presence of a resisting medium in space, we can only 
hope to obtain satisfactory evidence by investigating its effects 
on planetary motion, and comparing our deductions with ob- 



m 

served phenomena. Accordingly, about the close of the year 
1852, I was led to trace the fate of a satellite, which, in con- 
sequence of a continual loss of motion during innumerable 
years, was brought so close to the central sphere that it could 
no longer retain the planetary form. I arrived at the conclu- 
sion that the matter of the dilapidated world would ultimately 
arrange itself in an annular form around the primary, some 
time before I suspected that our solar system could furnish an 
evidence of so extraordinary a change. On comparing the ar- 
rangement which the dismembered mass would ultimately 
assume in its rapid evolutions, with the conditions of Saturn's 
ring, I was impressed with the belief that I had found a clue 
to the nature and the origin of this curious appendage. An 
inquiry respecting the stability of satellites placed so close to 
the primary furnished results favorable to my anticipations. I 
found that at the mean distance of the outer ring, a satellite as 
dense as Saturn would be incapable of maintaining its planetary 
form, nor could one of double the density revolve in security in 
the region which the center of the inner ring occupies. It soon 
became apparent that if fluid or solid satellites were originally 
placed on this dangerous ground, or were introduced into it by 
the resistance of a medium, they must undergo a dismember- 
ment and be ultimately converted into two rings. I was thus 
led to conclude that the existence of rings around Saturn is to 
be ascribed to their proximity to the planet, and that they are 
composed of the matter of two former satellites. 

These views, after occupying my attention for about seven 
months, were made the subject of a paper for the Cleveland 
meeting of the American Association for the Advancement of 
Science. The paper, however, was not read at the meeting, 
and I found considerable difficulty in procuring an examina- 
tion of the mathematical principles on which my theory de- 
pends ; but I was afforded an opportunity of bringing it before 
the Scientific Convention at Albany, in 1856. So far as I 
have been able to learn, the doctrine contained in my paper, 
and read at that meeting, and published in the proceedings 
(pp. 111-113), has not hitherto been controverted. 



THE KINGS OF SATCKN. 43 



THE RINGS OF SATURN. 



The Saturnian system seems to present a specimen of celestial 
architecture wholly different from anything yet observed in 
other departments of space. The great planet is surrounded 
by two vast zones of matter, or rings, which have a thickness 
not exceeding one hundred miles, while some estimates reduce 
it to even one-third of this amount. The outer ring has a 
diameter of one hundred and seventy-two thousand miles, a 
breadth of nearly eleven thousand, and the area of both of its 
sides is over fifty times that of the earth's surface. The inner ring 
contains even a greater area : its exterior diameter being one 
hundred and forty-eight thousand miles, and its breadth about 
seventeen thousand. Besides a permanent division of two 
thousand miles between both rings, temporary openings of vast 
magnitude have been detected in it on various occasions, and 
show a want of the state of repose which is found necessary to 
render our lands a suitable habitation for so many myriads of 
living beings. 

While observations have contributed much to invalidate the 
opinion that Saturn's rings are two integral, solid masses, 
physical inquiries tend to prove the extreme difficulty, ; f not 
the impossibility of maintaining such vast solid structures in so 
peculiar a condition. As their great size must render them 
utterly incapable of sustaining their own weight, astronomers 
have concluded that they must revolve around Saturn ; but 
even a revolution sufficiently rapid to equipoise the attraction 
of this body would not be adequate to avert their destruction. 
If we suppose, with many astronomers, that all parts of the 
inner ring wheel around Saturn once in about ten hours ; this 
movement, though deemed best suited for its preservation, 
would cause such a preponderance of gravity at the part next 
the primary, and of centrifugal force in the vicinity of the outer 



44: THE KINGS 

edge, that it must be torn asunder if it were one solid plate, 
composed of matter having one hundred times the tenacity of 
wrought-iron. The strain arising from the weight and the 
inertia of so vast an expanse of matter could not be materially 
diminished by the attractive power of the ring itself, unless it 
has a far greater mass than is assigned to it by the determina- 
tions of Bessel. According to the estimate of this astronomer, 
both rings contain less matter than that which composes our 
globe, and from this it is evident that, though they were strong 
enough to remain unbroken, they could not retain fluids or 
disconnected solid mass, except in a very narrow zone extending 
along their central regions. 

If the rings be divided, as some suppose, into a number of 
narrow ones, each having a motion adequate to counterbalance 
Saturn's attraction, the inevitable collisions between these 
annular structures would soon reduce them to ruins. But this 
is not the only danger that threatens their existence. Laplace 
has shown that, to prevent a solid, inflexible ring from striking 
the primary, there must be concentrated at some part of the 
periphery a large excess of matter which serves to control its 
movements. Now if such an excess of matter were stationed 
on any part of a single ring, it would become the center of an 
excessive attraction, and the passive strength of the vast 
structure could not resist the tendency to form a satellite from 
the entire mass. If a system of narrow rings were similarly 
constituted, their mutual disturbances would be augmented, 
and their collisions rendered more certain; so that the device 
of Laplace for securing them from the effects of Saturn's attrac- 
tion would expose them to greater danger from their own. 
Even a single narrow ring of such vast extent would be torn to 
pieces by the attraction of the satellites, or the inequality of 
the orbital motion of its own parts. We may, therefore, con- 
clude that Saturn's girdle must be composed of fluid or inco- 
herent, solid matter, so that each part may be permitted to 
acquire the velocity necessary to counterbalance the attraction 
of the planet, and may readily accommodate its form to the 
forces to which it is subjected. 

Though the hypothesis of the fluidity of the rings, as ad- 



OF SATUEN. 45 

vanced by Mr. Bond, accounts for their peculiar condition, it 
would be going too far to maintain that no other kind of mat- 
ter would be capable of forming such an appendage around a 
planet. A numerous host of fragments, resulting from the 
wreck of a satellite, or any other cause, would arrange them- 
selves in a similar form, if situated close enough to the pri- 
mary, and, after repeated collisions should bring their parts 
into a minute state of subdivision, we may expect a very close 
approximation to the form which fluid matter should assume. 
It has been urged as an objection to the latter hypothesis, that 
if the ring had a fragmentary constitution, each fragment must 
have others revolving around it. But such revolutions, instead 
of being necessary, are absolutely impossible in such close 
proximity to the great central body, whose disturbing force 
would prevent their mutual attraction from confining them to 
their small orbits. 

But though neither theory nor observation may enable us to 
form any definite conclusion respecting the composition of 
Saturn's ring, it may be considered as certain that the matter 
composing it must have so little coherence that each part moves 
independently with a velocity depending on the size of its 
orbit. The velocity of each zone of the whirling mass main- 
tains very nearly an inverse ratio to the square root of the 
distance from the central body, and the nearest part of the 
inner rinse makes about three revolutions before the most dis- 
tant part can complete two. This doctrine may, perhaps, be 
unfavorable to the contemplations of many astronomers, who 
regard the rings as fiat worlds, capable of supporting a popu- 
lation of rational beings far more numerous than the human 
race. But in such a habitation, all vital forms must suffer the 
greatest inconvenience, if not the most fatal evils, from the 
great length of the days and nights, as one side of the ring 
is exposed to the direct rays of the sun for nearly fifteen years 
without interruption, and deprived of their cheering influence 
for an equal period. Indeed, if the numerous advantages 
which organized beings require for their development could be 
furnished by such cosmical structures, we may have more reason 
to regret that the greater part of the matter in the solar sys- 



4:6 THE KINGS 

tern has not been molded into this form, as it would thus 
acquire an increased surface, and be rendered capable of 
sustaining more inhabitants. 

A fluid or a fragmentary ring, notwithstanding the appar- 
ently chaotic condition which it may present from the want of 
uniformity in the movement of its several parts, seems des- 
tined to fulfill higher purposes in the economy of creation. It 
is well known that heat is the offspring of mechanical action, and 
it has been recently found that the agitation of water, and even 
the flow of rivers is attended with an elevation of temperature. 
Now, whatever be the proportions of solid and liquid materials 
in the ring of Saturn, the movement of its several parts with 
different velocities must be attended with considerable friction, 
and this becomes a fertile source of heat. The effect is in- 
creased by the pressure of the contiguous zones of matter, as 
they are driven one against the other in consequence of the 
slight eccentricity which even the attraction of the satellites might 
occasion. Accordingly the Saturnian system, although far 
away from the sun, is not abandoned to the sway of intolerable 
cold. The high temperature arising from the attrition of the 
several parts of the ring, and their mutual collisions, must 
be sufficient, not only to keep much of its matter in a fluid 
condition, and to clothe it with an atmosphere of vapor, but 
even to act an important part in warming the great planet, 
and establishing a temperate climate in the frigid zone of the 
solar system. 

How far the superior brightness of the ring is dependent on 
this cause it would be difficult to determine. The rays of light 
have much of their illuminating power exhausted in producing 
heat on passing through the air or gases, and accordingly the 
presence of a dense atmosphere would make a planet display 
less brilliancy to the inhabitants of other worlds. From the 
planet Venus, the moon would appear somewhat brighter than 
the earth, and it is evidently from a deficiency of their aerial 
envelopes that Jupiter's satellites have a relative brightness 
superior to that of the primary. But clouds prevent th e 
solar rays from wasting their power by passing into the dense 
air ; and accordingly, as William Herschel maintained many 



OF SATURN. 47 

years ago, cloud surfaces reflect a more intense light than 
the rest of a planet. Although Saturn may appear less lumi- 
nous than his ring, in consequence of being able to confine a 
more dense atmosphere around him, yet the changeable 
brightness of the latter can only be ascribed to a heating 
power peculiar to itself. From this cause evaporation is ren- 
dered more copious, and the consequent formation of clouds 
tends to maintain the continuity of the ring, and to reflect more 
light to distant worlds. Even the planet itself exhibits indica- 
tions of atmospheric movements, and a greater variation of 
climate than the sun could produce from such a vast distance, 
and he must evidently receive considerable supplies of heat 
from some other source. 

The production of heat in tlfe ring necessarily involves the 
destruction of living force in the annular mass, and a corres- 
ponding change in the orbits of its several parts. According 
to Hansen and other astronomers, the distance between Saturn 
and the inner edge of his nearest ring has diminished many 
thousand miles during the past two centuries, but it is probable 
that the estimate is too high. Supposing the change took place 
so slowly that fifty thousand years should elapse before the pre- 
cipitation of the last part of the rings to the planet, the me- 
chanical action alluded to would develop, during that period, 
a greater amount of heat than could be produced by the hourly 
combustion of 3,000,000,000,000 tuns of coal. The amount 
of heat produced on the sun's surface is, according to the es- 
timate of Pouillet, over 10,000,000 times greater, but, in 
consequence of its great distance, it exerts a far less decided 
influence on the temperature of this remote planet. 



48 SUPPOSED INFLUENCE OF SATELLITES 



THE SUPPOSED INFLUENCE OF SATELLITES IN PRESERVING 
PLANETARY RINGS. 



In the foregoing pages it Las been shown that the existence 
of rings around any planet is the necessary consequence of the 
limited density of their materials, and the proximity of the 
central body. While astronomers have shown much concern 
for the safety of Saturn's girdle, it seems to be the only form 
which the matter could assume in such a close proximity to his 
surface. The disturbing action of the primary renders it im- 
possible for the annular mass to change into satellites, and there 
is still less danger that any portion of it should come into a pre- 
mature collision with the central sphere. This could only 
take place by the continual increase of the eccentricity of the 
orbits of the several parts of the ring, and these orbits are far 
less exposed to such a change than are the paths which single 
planets or satellites describe. 

Professor Pierce, of Cambridge, while adopting the views 
of Mr. Bond, contends that even a fluid ring could not be 
maintained around Saturn were it not for the presence of his 
satellites, and that a proper number of such attendants is essen- 
tial to the existence of a ring around every planet. Though we 
must regard views emanating from so high a source as wor- 
thy of consideration, we must dissent from many persons who 
represent them as supported by a mathematical investigation. 
In the case of Saturn, there is certainly no adequate basis 
for such an investigation, for the masses of his satellites are 
unknown, and therefore no special effect depending on their 
action can be calculated. If the stability of the rings were 
independent of the masses of these bodies, it would exist if 
they were extremely small, or even reduced to nothing, and 
there is accordingly no reason to believe that the removal of 
the satellites would cause the great annular ocean to dash 
against the planet or to roll itself up into satellites. 



IN PRESERVING PLANETARY RINGS. 49 

The supposed agency of the satellites is ascribed by Pro- 
fessor Pierce to the periodical character of the disturbances 
occasioned by their attraction, but the theory of planetary 
motions only proves that this cannot be attended with any 
positive conservative influence. Lagrange and Laplace have 
indeed shown that the mutual disturbances between the several 
planets are characterized by such a periodicity that they 
counteract at one time the effects they produce at another, 
and that they accordingly fail to destroy the stability of our 
system, or to make any permanent alteration in its condition. 
Our earth is thus secured from the effects which more stupen- 
dous spheres may be expected to produce; but it would enjoy, 
at least, equal security if it were the only planet attending the 
sun. Physical astronomy only shows a negative influence, 
resulting from mutual action, so far as stability is concerned. 
We may therefore conclude that Saturn's ring is secure from 
the dangers of the attraction of his satellites, in consequence 
of the periodicity of their disturbing forces ; but it cannot 
derive from these disturbances any degree of safety, or security, 
which did not exist in their absence. 

But the most serious objection to the theory in question is 
revealed, when we examine the adequacy of the cause to the 
task which it is supposed to perform. If Venus and the earth 
moved in the same plane, and nearly at the same mean dis- 
tance from the sun, the disturbances which both must ex- 
perience from mutual attraction would be exceedingly great, 
and the consequences to which it might lead could not be 
averted by the influence of more distant planets. Now if, as 
Professor Pierce supposes, each part of the ring is to be 
regarded as a satellite, the attraction between the parts which 
move in orbits, differing little or nothing in size, must give 
rise to disturbances enormously large in comparison to those 
proceeding from the eight moons which roll at far greater dis- 
tances from the planet. It would be absurd to expect that the 
action of the latter bodies is capable of counteracting effects 
several hundred times greater than any it can produce ; or that 
it can oppose any effectual resistance to the power which tends 
to congregate the matter of each ring into a single satellite. 
4 



50 SUPPOSED INFLUENCE OF SATELLITES 

From the relation between the size of the orbits of the 
several parts of the ring, and the friction and other impedi- 
ments with which they have to contend, we cannot ascribe to 
their disturbing influence the periodicity which occurs in 
planetary systems. The ring cannot, therefore, preserve itself 
from the attraction of its own matter, and we must abandon 
the idea that an annular group of satellites could revolve in 
the present lunar orbit with perfect security. Such an assem- 
blage of diminutive moons would soon form a single mass, 
unless the force of gravity between them was entirely sup- 
pressed, or deprived in some way of its concentrative power. 
Now this could not be possible if they were as distant from us 
as our present satellite ; but if their orbits were not more than 
sixteen thousand miles in diameter, and their density not greater 
than that of granite, their attraction would be suppressed in 
two directions by the agency of terrestrial gravity, and they 
would be thus rendered incapable of congregating into larger 
masses. 

As the rings of Saturn are in the same degree of relative 
proximity to him, he must exert a similar influence in neutral- 
izing the gravity of their matter in two directions, and thus 
effectually prevent it from changing into secondary planets. 
The existing satellites can have only an imperceptible control 
over this gravity ; and though they may cause tides in the great 
annular ocean, these can never attain such a magnitude as to 
act the part of the irregularities which Laplace finds necessary 
for the security of a solid ring. Even were the tides exhibited 
on a scale sufficiently lar^e, and the conclusions of Laplace 
free from objections, it would be wrong to suppose that a fluid 
ring, and one of unyielding solidity, would be affected in the 
same manner by an excess of matter accumulated on any 
part of its surface. 

From the instability of planetary bodies revolving in small 
orbits, it appears they could never be formed from the matter 
composing Saturn's ring, though Professor Pierce supposes that 
such an event might take place if his present satellites were 
removed. He also maintains that, in such a case, a collision of 
the rings against the primary would occur ; and his grounds 



IN PRESERVING PLANETARY RINGS. 51 

for this conclusion I must briefly notice. In its movements 
around the planet each part of a fluid ring must expand in pro- 
portion as its velocity became slow ; and, according to Pro- 
fessor Pierce, this enlargement takes place in such a manner 
as to reduce the mighty stream to a state of unstable equili- 
brium, and leave it at the mercy of every external disturbance. 
In his own words, " an exact analysis shows that the accumula- 
tion of fluid in any part precisely balances the greater distance, 
and hence the ring is equally attractive in every direction. 
The regulating action upon the motion of the center of gravity 
of the ring is therefore canceled, and it must continue to move 
uniformly in any direction in which it may be started b} r any 
foreign influence." The first assertion would be correct if 
Saturn were an exact sphere, and the matter of his rings were 
sensitive to no power except his attraction. In this case every 
part of the numerous anuuli into which the fluid divides would 
describe areas proportional to the time ; and the accumulation 
of matter at any point would vary at the same rate as the 
square of the radius vector. Taking this in connection with 
the fact that the gravity of spheres is inversely proportional to 
the square of the distance, it would be easy to see that the 
ring exerts the same attractive power on the planet in every 
direction. But the result is different when we take into con- 
sideration the spheroidal form of Saturn, the attraction between 
the several parts of the ring, and the impediments to their 
motion arising from friction and other causes. If the ring was 
started in one direction by any disturbance, or if all its parts 
described ellipses, having their transverse axes directed to the 
same point, a powerful tangential force would come into play, 
the angular velocity could be no lunger as inversely propor- 
tional to the square of the radius vector, and a great apsidal 
movement would be exerted in rendering the ring concentric 
with the planet. 

The conclusion expressed in the last part of the quotation is 
still more objectionable. Whatever be the state of attraction 
between both bodies, the ring could be made to move uniformly 
to one side of the planet, only by having the orbits, which its 
several parts describe, constantly elongated in the same direc- 



52 SUPPOSED INFLUENCE OF SATELLITES 

tion, and this effect could not proceed from any known cause 
operating in the celestial regions. A constant change in the 
eccentricity of a planetary body requires that the disturbance 
which produces it should be constantly repeated. If the 
Saturnian system were exposed to a disturbing force which 
always maintained the same direction from the central body, 
it would continually increase the eccentricity of the orbits of 
his satellites ; but the paths which the several parts of the 
rings pursue could not be affected in the same manner, as 
their circular character would be maintained by the causes 
already alluded to. In supposing a case which cannot occur 
in nature, my object is to show that planetary rings are more 
secure than satellites from premature catastrophes. 



THE MOVEMENTS OF COMETS. 53 



THE MOVEMENTS OF COMETS. 



The limited means which astronomy has hitherto furnished 
for exploring the heavens, have been sufficient to reveal a great 
diversity in the size andphysical character of the celestial bodies, 
even within the confines of our solar system. The large pri- 
mary planets, of which our earth can be scarcely considered 
a fair specimen, form only a very small part of the bodies 
which attend the Sun and obey his attractive power. A nu- 
merous family of asteroids occupy the region between Mars 
and Jupiter, while a still greater number of comets are per- 
mitted to pursue their wandering courses through a more ex- 
tensive department of the planetary domain. The second-rate 
worlds and rings which attend the larger planets, exhibit on a 
more imposing scale, the great variety which the wide realms 
of space are capable of affording. But while the telescope 
gives intelligence of the existence of large or conspicuous 
celestial bodies, it fails to detect a more numerous class, which 
have been lately recognized by astronomers as humble mem- 
bers of the solar family. Meteoric stones and shooting stars 
are now almost universally regarded as small cosmical masses, 
revolving around the sun, until they come in collision with 
some of the planets and incorporate with them forever. 

As the physiologist, instead of confining his studies to the 
organization of man, or of the higher species of the brute crea- 
tion, often finds it to his advantage to investigate the phenom- 
ena of life in its most simple forms, so the astronomer must 
give equal scope to his investigations respecting the occupants 
of space. In the larger spheres he sees an adaptation to certain 
great ends, while the celestial bodies of a more humble char- 
acter, though deprived of similar advantages, yet reveal by their 
movements the unswerving operation of the wonderful power 
which holds the material universe together. The wandering 



54 THE MOVEMENTS OF COMETS. 

bodies which have so often excited the curiosity, the wonder, 
and the fears of mankind, will form the subject of the present 
inquiry. 

Comets appear to be composed of exceedingly rare nebulous 
matter, somewhat condensed around a point called the nucleus, 
as if it were compressed by the attraction which holds its parts 
together. The more conspicuous of these curious bodies are 
characterized by the presence of a tail, which is chiefly re- 
markable for the extraordinary length it frequently attains, 
as in some instances they have been known to extend, over a 
distance of more than 100,000,000 miles. These vast trains 
gradually increase. in size during the approach of the comet to 
the Sun, and continually decline on its departure to the fron- 
tiers of our planetary system. The other part of the come- 
tary mass appears to have its size reduced in the vicinity of 
the solar orb, and this fact has served to produce the impres- 
sion that the head is exhausted in the formation of the tail. 
This opinion, however, does not seem to be countenanced by 
any information which has been yet obtained respecting the 
forces prevading the realms of nature. 

It is only by a comparison with the objects around us that 
we can be enabled to form any ideas respecting the constitu- 
tion of the celestial bodies, and the atmosphere which invests 
our globe seems to afford the nearest resemblance to cometary 
matter. Similar atmospheric envelopes surround other planets ; • 
but in all, they are very insignificant in quantity, if compared 
with the denser liquid or solid matter of which the planetary 
structure is chiefly composed. On the contrary, in cometary 
bodies, the aeriform elements have a similar preponderance, 
and the proportion of solid or liquid matter is so small that 
many astronomers entertain doubts respecting its existence. 
Even the vast expanse of nebulous matter which composes the 
head of these bodies is so much more attenuated than the air 
we breathe, that our comparison can be scarcely considered just, 
and must be regarded more as a means of illustration than a 
basis of scientific inquiry. 

We must not, however, suppose that our experience with 
terrestrial objects is wholly inadequate to furnish a clue to the 



THE MOVEMENTS OF COMETS. 55 

mysterious phenonomena exhibited by these wanderers of 
space. All material forms are constrained to obey certain 
fixed laws, and of these the law of gravity has been brought 
within the domain of accurate investigation. In an exhausted 
receiver, a feather and a piece of gold descend with the same 
velocity, and in the more perfect vacuum of the celestial spaces, 
the attraction of the sun manifests the same impartiality in 
moving and controlling the motion of all bodies, whether 
they be planets, comets, or meteoric stones. Accordingly, 
without knowing anything respecting the density of these 
bodies, or their composition, we may determine their move- 
ments from the great law of universal gravitation. 

The first step to a solution of this important problem may 
be ascribed to the celebrated Galileo, who undertook to calcu- 
late the path which projectiles would acquire from their 
motion combined with the earth's gravity. In an age when 
the use of gunpowder afforded the means of imparting such a 
high velocity to cannon-balls, it became an object of interest to 
determine the nature of the curve which they describe in their 
flight through the air. From the extreme difficulty of calcu- 
lating the effects of aerial resistance, the theory of gunnery is 
of little use to the practical artillerist, but the results which 
might be expected, in the absence of the air, can be readily 
determined. In this case a cannon-ball projected with a 
velocity of eight hundred feet a second, and at an elevation of 
thirty degrees, would pass over nearly four miles of a horizon- 
tal plane. If the velocity were doubled, the range would be 
nearly sixteen miles. Were it not for the presence of the air, 
the path of projectiles would be nearly a parabola, and their 
ranges nearly proportional to the square of the initial velocity. 

Convinced thai the laws of motion must apply to bodies 
moving with the greatest velocities, Newton resolved to extend 
the investigations which Galileo commenced. As the path of 
a projectile is expanded by every increase of velocity, it would 
be reasonable to expect that it would ultimately be commen- 
surate with the earth's curvature, and that a body could be 
sent quite around our planet by an impulse of adequate energy. 
A horizontal velocity of five miles a second, imparted to a ball 



56- THE MOVEMENTS OF COMETS. 

just beyond the limits of the atmosphere, would cause it to 
complete a circuit of our globe and continue revolving around 
it as a satellite. Such a projectile would indeed be deflected 
from a rectilinear course at the rate of about sixteen feet every 
five miles ; yet it must still remain over the earth's surface, 
which has as great a deviation from a straight line. 

In consequence of our utter inability to impart such high 
velocities, or to ascend beyond the limits of the air, we can. 
never expect to verify, by direct experiment, the profound 
deduction of Newton. Had our lot been cast on an asteroid, 
one hundred miles in diameter, and as dense as the earth, we 
might be in a condition for exhibiting the mechanism of the 
heavens on a small scale, or, at least, we could perceive a far 
greater resemblance between the course of projectiles and the 
movements of the great spheres which compose the solar 
system. It would require a horizontal velocity of only three 
hundred and thirty feet a second to send a body quite around 
such a diminutive world, from the summit of any of its 
mountains which rose above its atmosphere. Had the velocity 
been four hundred feet a second, it would sweep ninety-eight 
miles beyond the opposite side of the little planet ; but it would 
return to the place from which it started. If the initial velocity 
were four hundred and fifty feet a second, it would wander about 
nine hundred miles farther into space ; and if it were as high as 
five hundred feet a second, the body would never return to the 
sphere from which it had departed. It may be shown, by a 
more difficult mathematical investigation, that the orbit would 
be either a circle or an ellipse, if the velocity were confined 
within certain limits ; but, with an excess of velocity, a body 
may be caused to describe a parabola or a hyperbola. 

The attractive force of the sun is so powerful, even at great 
distances from his surface, that an inconceivable amount of 
motion is required to control it, and accordingly, planets and 
comets must move very rapidly to maintain their condition in 
the solar system. To cause a ball, or a small planet, to describe 
a circle around the sun, at the distance of ninety millions of 
miles from him, it should receive a velocity of seventy thousand 
miles an hour, in a direction perpendicular to that of the cen- 



THE MOVEMENTS OF COMETS. 



57 



tral luminary. If the primitive impulse was sufficient to move 
it at the rate of eighty thousand miles an hour, the body would 
describe an elliptical orbit, stretching 170,000,000 miles beyond 
the opposite side of the sun; and a velocity of 91,000 miles an 
hour, would cause it to sweep as far as the region of Jupiter. If 
it started at the rate of ninety-five thousand miles an hour, it 
would pass beyond the orbit of Saturn; while a velocity of 
ninety-seven and ninety-eight thousand miles an hour would 
send it as far as the orbits of Uranus and Neptune. A slight 
increase to these high velocities would give the centrifugal force 
a perpetual predominance over the solar attraction ; and were 
the body in question originally impelled at the rate of ninety- 
nine thousand miles an hour, it would describe a hyperbolic 
orbit, and never revisit the sun. 

It thus appears that the very eccentric ellipses which comets 
describe, result from the action of the same force which gives 
almost circular paths to the several large planets. In the case 
I have introduced, the primitive impulse is supposed to be ex- 
erted in the tangent of a circle concentric with the sun. Any 
deviation from this directiou must be accompanied with a cor- 
responding eccentricity, even though the velocity be such as 
might confer a circular orbit under different circumstances. 
The very elongated paths which comets pursue were at one time 
regarded as an unaccountable phenomenon in celestial move- 
ments, and even the cotemporaries of Kepler were much aston- 
ished to learn that the planets did not describe true circles. At 
the present day, when the theory of celestial motion is under- 
stood, the chief cause of surprise has been, not that the orbits 
of comets are so eccentric, but that those of the planets should 
approximate so closely to a circular form. 

From the peculiarities of the paths which comets describe, 
there is a very great disproportion between their greatest and 
least distances from the sun — many of them sweeping within 
the orbit of Mercury, and then retiring beyond the region of 
the most remote planet. The comet of 1680 almost grazed 
the sun's surface, while its greatest distance from him, accord- 
ing to the calculations of Encke, is 70,400,000,000 of miles. 
It is only during the brief period that they spend traversing our 



38 THE MOVEMENTS OP COMETS. 

system, that these celestial adventurers are visible, and in this 
part of their course they move with very nearly the velocity 
which a planet would require to describe a circular orbit at half 
the distance from the sun. In approaching from remote space to 
the central part of the solar system, their motion is constantly 
accelerated by the attraction of the central orb, until they ar- 
rive at the perihelion point and commence their retreat. In 
its perihelion passage, Halley's comet moves at the rate cf 
125,000 miles an hour, while the comet of 1680, and that of 
1843, must have over nine times this velocity at the same part 
of their orbits. 

On retiring from the interior of our system, comets have 
their motion constantly retarded, until it sinks to its lowest 
state, when they reach their aphelion, or greatest distance from 
the sun. The aphelion velocity of Halley's comet does not 
exceed 2,000 miles an hour, while that of Encke's is nearly 
seven times as great. But if we can rely on Encke's estimate 
of the aphelion distance of the comet of 1680, this body must 
move at the rate of only ten feet a second in the most distant 
part of its orbit, thus exhibiting a wonderful contrast with its 
terrific velocity in sweeping through the central regions of our 
system. 

But it may be necessary to state that calculations for deter- 
mining the distance to which these bodies retire into space can- 
not have any pretensions to great accuracy. A very slight 
change in their perihelion velocity would enlarge or dimmish 
to an enormous extent the dimensions of their orbits, and would 
be attended with a corresponding great alteration in the times 
of their revolution. In like manner the very trifling errors, 
which are inseparable from observations made with the best 
instruments which the astronomer can command, would cause 
his estimates of the aphelion distance to differ very widely 
from its actual value. Accordingly, notwithstanding the great 
numbers of these bodies which have been observed, astrono- 
mers have succeeded in determining the orbits of only a few, 
and of these the comets of Halley, of Biela, and of Encke, seem 
to be most deserving of consideration. 

Encke's comet is remarkable for having a smaller orbit and 



THE MOVEMENTS OF COMETS. 59 

•a more brief period of revolution than any body of the kind 
yet discovered. The average length of this period is about 
1,204 dt.ys, and it is now about two days less than it was sixty 
years ago, when the comet was first observed. This change 
Encke has ascribed to the presence of a rare fluid disseminated 
through space. In moving through an absolute vacuum, an 
attendant of the sun would sustain no permanent change in its 
velocity, but the effects of a resisting medium, in destroying- 
its motion, would give it a smaller orbit, and cause it to move 
more rapidly, in obedience to the more powerful attraction of 
the central orb. The great rarity of cometary bodies renders 
them more sensitive to the impressions of this fluid, but from 
the superior density of the planets, the same resistance could 
not perceptibly change the dimensions of their orbits during a 
million of years. 



60 THE TAILS OF COMETS. 



THE TAILS OF COMETS. 



While the more humble cometary bodies present compara- 
tively little to excite our astonishment, the more conspicuous 
members of this class of celestial objects derive much import- 
ance from the immense tails which extend from them in the 
presence of the sun. These singular appendages, commencing 
at the body of the comet, stretch their immense lengths several 
millions of miles into space, maintaining almost invariably a 
direction directly opposite to that of the sun. From their ex- 
traordinary size and the vast extent over which they sweep, it 
is scarcely possible that they could always avoid coming in 
contact with the several planets, and the consequences of such 
an event have frequently been made the subject of speculation 
to astronomers. Whiston went so far as to maintain that it was 
the tail of a comet that supplied the waters of the deluge, and 
even in more recent times, astronomers have been impressed, 
with the idea that a collision with one of these long cometary 
appendanges may seriously affect the composition of the atmo- 
sphere. 

It will be seen, however, that there is little just ground for 
these apprehensions. The tendency of the human mind to 
judge too hastily from appearances has led to the belief that 
these visible trains must be composed of cometary matter. 
The more careful researches of modern times, however, tend 
to shake this opinion, by showing that they are entirely devoid 
of the leading characteristics of material bodies. While the 
nucleus and envelop of cometary masses have their movements 
directed by the laws of gravity, the tails pay no regard to the 
government of this power. The parts most distant from the 
sun have always the most rapid angular motion, while the at- 
traction of this body should call for the greatest velocity in the 
greatest proximity to him. In turning the perihelion, the move- 



THE TAILS OF COMETS. 61 

merit of the tail shows an entire freedom from the restraints of 
material laws. A good illustration of this was furnished by 
the comet of 1843, which swept over the sun's disk at the rate 
of 300 miles a second, and extended its tail as far as the orbit 
of Mars. The extreme end of this appendage, to keep directed 
to the sun, must have traveled over 200 times as rapidly as the 
nucleus, and about 3,000 times faster than the earth in describ- 
ing its annual orbit. 

Finding that the effects of gravitation fail to account for such 
anomalous movements, astronomers have been generally dis- 
posed to ascribe a somewhat different character to the phenom- 
enon. According to Euler and Delambre, the tail is composed 
of materials separated from the comet by the impulse of solar 
rays, and Herschel modifies the theory by ascribing the sepa- 
ration to the electrical repulsion of the sun. But, to form a line 
constantly ranging with the central luminary, the matter should 
be driven into space with a far greater velocity than the comet 
has in describing its orbit, and the sun could scarcely commu- 
nicate the requisite centrifugal motion if his repulsive power 
were one hundred thousand times as powerful as his attraction. 
Now the movements of the several members of the solar sys- 
tem show that the repelling power of the central orb (if it has 
any existence) must be extremely feeble in comparison to his 
attractive power; and no astronomer has hitherto found it nec- 
essary to take into consideration the slight influence which it 
exerts even on cometary orbits. 

Modern chemistry has revealed some properties of light 
which may give rise to the extraordinary appearances attend- 
ing the returns of comets to the interior of the solar domain. 
Among the numerous chemical agencies which we are permit- 
ted to command, the solar rays hold a prominent place, and 
their power of effecting decompositions and inducing chemical 
changes is modified to a considerable extent, not only by the 
bodies which reflect them, but also by the character of the 
medium through which they are transmitted. Sulphate of qui- 
nine, and many other fluids, are found capable of altering the 
nature and the chemical efficiency of the light which passes 
through them, weakening some of the rays and rendering 



62 THE TAILS OF COMETS. 

others more energetic in their action. In such cases it must 
be observed that these media after a time fail to influence the 
character of the light, and it would seem that their properties 
are impaired by the work they are called to perform. 

These and other analogous results have led me to the follow- 
ing conclusion, which I subsequently found to be confirmed by 
many striking facts. The virgin matter of those comets which 
comes from remote realms of space to the vicinity of the sun 
must be very sensitive to the influence of his light, and the 
rays which pass through the envelope must experience a very 
great change. Their chemical power would evidently be in- 
creased by the separation of the calorific rays, and probably by 
the play of affinity in the materials, as the temperature is ele- 
vated. If space contained any matter which might serve to 
test the chemical energy of two kinds of light, the difference 
between the rays sent through the comet, and those traversing 
the rest of the domain, would be visibly inscribed in the skies. 
]S T ow it appears from the secular change in the orbit of Encke's 
comet, and from other facts, that a rare medium is disseminated 
through the inter-planetary regions ; and it will be shown, in 
another chapter, that this consists not only of a fluid capable 
of conveying light, but also of a luciferous ether, which sus- 
tains the perpetual brilliancy of suns. It is on this fine matter 
that the light transmitted through the body of a comet exerts 
its action, inducing changes of a luciferous character, and form- 
ing the tails, or visible trains, which these bodies cast behind 
them. There is, accordingly, no more necessity for regarding 
these appendages as composed of cometary matter, than there 
is for supposing that a photograph partakes of the composition 
of the object which it has been enabled to represent, by a sim- 
ilar mysterious agency of the solar rays. 

To produce more direct evidence in regard to this theory, I 
shall now show that even the light of distant suns, on passing 
through the comets of our system, is rendered capable of call- 
ing forth the light-yielding powers of the ether of space. It 
was long a matter of surprise that the vast expanse of nebu- 
lous matter surrounding the nucleus, could not sensibly obscure 
the brilliancy of the stars over which it passed ; but it has 



THE TAILS OF COMETS. 63 

been considered as still more unaccountable, that, in a few in- 
stances, there has been an augmentation of stellar brilliancy 
on such occasions. A star seen by Piazzi, through the head 
of the celebrated comet of 1811, was found to have its luster 
considerably augmented ; and a similar case was presented 
more recently, when a star of the eighth magnitude became 
as bright as one of the sixth, during the passage of Broisen's 
comet over it. These phenomena can only be accounted for 
by supposing that the rays of stellar light, after passing through 
the cometary matter, have their illuminating power consider- 
ably augmented by the luciferous action they set up in the 
etherial fluid, which they meet before arriving at the Earth. 

If the nebulous matter of comets had merely the power of 
reflecting and of intercepting solar and stellar light, without 
being capable of changing its character, in the manner I have 
described, these bodies would always obscure the brilliancy, 
and diminish the apparent magnitude of the stars over which 
they pass. The illumination of our atmosphere by the full 
moon, is sufficient to render stars, even of the fifth magnitude, 
incapable of producing any impression on the unassisted eye. 
According to the calculations of M. Babinet, a comet as laro-e 
as the earth, and having a density 45,000, 000, 000,000, 000 
times less than that of the ordinary air, would produce a more 
decided diminution of stellar brilliancy than has been hitherto 
found to aris3 from the presence of these rare bodies. From 
one of the most recent estimates of this astronomer, it would 
appear that " if a single cubic inch of our air could expand so 
as to fill a sphere, having a diameter one million times as great 
as the distance of the nearest fixed star, it would still be far 
more dense that the matter composing the comet of 1825. It 
will be seen, however, in the next chapter, that the mean den- 
sity of cometary materials, cannot be even a million times less 
than that of our atmosphere ; and that the great comet of 1 825» 
instead of having less than a grain of matter in its entire com- 
position, as the estimate of Babinet would lead us to suppose, 
must have contained several millions of tons. The results at 
which he has arrived, in regard to the inconceivable levity of 
these wandering bodies, must be several million times below 



64: THE TAILS OF COMETS. 

the actual value. While we admit the justness of his conclu- 
sions in regard to the diminution of brilliancy which stellar light 
undergoes from the intervention of the illuminated nebulous 
mass of comets, we must conclude that the effect is counter- 
balanced by the accession to their illuminating power, arising 
from their action on the etherial contents of space. 

But the overpowering influence of the solar rays alone is 
capable of producing, in the realms of ether, those visible trains 
which attend the most conspicuous comets. The part which 
solar light takes in their formation, is pioved very decidedly 
by the existence of the dark line which often extends through 
the central part of the tails of comets. It would seem that the 
nucleus, and the more dense nebulous matter which surrounds 
it, either intercept the solar beams, or fail to impart to them 
the power of depicting their characters on the celestial regions. 
The appearance of the tails generally corresponds to what a 
vast hollow cone of luminous ether might be expected to pre- 
sent. Had they been formed of cometary matter, it must have 
been all driven into space with precisely the same rate of mo- 
tion, and its centrifugal velocity must have been little inferior 
to that of light. Even if these extravagant hypotheses were 
admitted, the explanation which they furnish would be very 
unsatisfactory. 

The great resemblance which these cometary trains bear to 
a negative shadow, has been recognized by astronomers. The. 
degree of curvature which they occasionally exhibit, seems to 
furnish some difficulties to the present theory of their origin, 
but a little consideration will show that a considerable devia- 
tion from the rectilinear direction must be expected from the 
aberation of the cause to which I have ascribed them. The 
operation of solar light must necessarily give rise to an appar- 
ent and actual curvature of the vast streamers which they form 
In space. This may be most easily understood from a refer- 
ence to the comet of 1680, which, at its nearest distance from 
the sun, described about two degrees of its orbit in the space 
of sixty-five seconds. Now, as light travels thirteen millions 
of miles in the course of sixty-five seconds, it is evident that 
the rays which the sun sent through the comet in this position, 



THE TAILS OF COMETS. 65 

must deviate about two decrees from the direction of those 
which were thirteen millions of miles in advance of them. — 
Supposing the tail to originate from their instantaneous action, 
it would necessarily deviate at this place two degrees from a 
rectilinear direction, in the course of thir ten millions of miles, 
and the deviation for the entire length would amount to over 
sixteen degrees. The aberration of light also causes each 
part of the cometary trains to appear in the region from which 
it has departed, and this will give it an additional degree of 
curvature, while even the position in which it is viewed from 
the earth has frequently a more decided influence on its ap- 
parent form. 

It may be, however, necessary to observe that this class of 
celestial phenomena furnishes some anomalous cases which will 
be considered in the succeeding chapter. A few cases have 
occurred in which comets have been attended with a plurality 
of tails, and their direction in some instances, differed very 
much from that of the'solar light. But in the cases in which 
they took these anomalous directions, they were always devel- 
oped on a very small scale, and evidently owed their existence 
to commotions in the envelope of the comet. It can hardly be 
doubted that great commotions exist in these bodies, though 
we may pronounce them inadequate to shoot matter off, with 
the velocity of light, to distant parts of the planetary domain. 
To judge of the effects which they are capable of producing, 
we must first endeavor to gain some idea of the mass and 
density of these wandering bodies. 
5 



66 MASS AND DENSITY OF COMETS. 



MASS AND DENSITY OF COMETS. 



r A definite knowledge of the quantity of matter which many 
of the celestial bodies contain, was among the earliest fruits 
which astronomers derived from the discovery of universal 
gravitation. In the second chapter of this work, I have shown 
how the relative weights, or masses of the sun, and some of his 
larger worlds, have been found from the effects of their at- 
tractive power in confining satellites to certain orbits. In other 
cases it has been determined from their influence in disturbing 
the orbital movements of neighboring planets. But to the more 
humble members of the solar system, who have no satellites to 
attend them, and whose attractive power is too feeble to exert 
any serious disturbing influence at great distances, these methods 
will not apply. The attempts to ascertain the weight of com- 
etary bodies have hitherto led only to negative results, by 
showing that it is too small to affect the delicacy of any astro- 
nomical balance which the ingenuity of science has hitherto 
devised. The comet of 1770 approached so close to the earth, 
that the most favorable conditions were afforded for a display or 
its attractive power, and if it had a mass equal to that of our 
planet, it must have diminished the length of our year one- 
ninth of a day. As the year has exhibited no perceptible 
change from the occurrence, Laplace concluded that five thou- 
sand such bodies could not furnish the amount of matter con- 
tained in our globe. 

But there seems to be placed within the reach of the astron- 
omer a far more delicate balance for determining the material 
contents of these celestial objects. Although the attraction of 
large spheres is frequently exhibited at the most immense dis- 
tances, it is always felt most powerfully by the matter of which 
they are composed, and in the lighter class of bodies we can 



MASS AND DENSITY OF COMETS. 67 

only estimate it by the effects it produces near their surfaces. 
Accordingly the feeble attractive power of comets, though not 
manifested in disturbing planetary motion, may be determined 
approximately from its capability of holding their parts together, 
while the action of the sun is exerted for their dismemberment. 
The levels of our oceans are perceptibly affected by solar attrac- 
tion, although they are restrained by a terrestrial gravity about 
twenty million times as great as the tidal action of the sun. 
If the earth, without changing its size, could have its density 
so much reduced that its gravitative force would be twenty 
million times more feeble than it is at present, bodies could 
have no weight at those places, in conjunction and in opposition 
with the sun, and the terrestrial materials, if they ever attained 
so rarefied a condition, would be incapable of preserving a 
planetary form. In this case, however, our planet would be 
more readily destroyed by its own rotation, or by the disturbing 
force of the moon ; but our illustration is merely intended to 
give the reader some idea of the influence of the sun on the 
stability of one of its rare attendants. - 

It is, therefore, evident that a comet as large as the earth, 
if describing a circle 190,000,000 of miles in diameter around 
the sun, could not resist his dismembering action, unless it were 
at least equal to the one twenty -millionth part of the terrestrial 
mass, or contained 300,000,000,000,000 tuns of matter. Had 
such a comet revolved in the orbit of Neptune, the attraction 
of 10,000,000,000 tuns of matter would be sufficient to main- 
tain its existence. The tidal action of the sun, or the disturbing 
action he exerts on the surfaces of his attendants, is inversely 
proportional to the cube of their distances, and the mean 
density which they require to maintain the planetary structure 
must vary in the same proportion. A comet having a mean 
density two hundred times less than that of the air we breathe, 
would be reduced to an unstable^condition, if it came as close 
to the sun as the planet Mercury, while describing the perihelion 
of its orbit. 

It must not, however, be inferred that all comets have a mass 
sufficient to keep their parts together, when they arrive in their 
closest proximity to the solar orb. In many cases there can be 



68 MASS AND DENSITY OF COMETS. 

little doubt that their envelopes yield to the effects of the sun's 
disturbing action in their perihelion passage ; but the great 
velocity with which they sweep through this part of their 
orbits, and the great density of their nucleii, must save them 
from a total dismemberment. The opinion, that cometary 
bodies sustain such a waste of their materials, has been prev- 
alent among astronomers since the days of Kepler, and it seems 
to accord with what has been observed in the constant diminu- 
tion of their envelopes on their approach to the sun. But as 
they are left in possession of copious envelopes, after numerous 
revolutions, and as they can not be supposed to have the power 
of collecting all of their lost matter from space, the loss which 
they sustain must be small. We may, therefore, conclude that 
during the principal part of a comet's revolution, the attraction 
w r hich holds its parts together is greater than the disturbing 
force which the sun exerts to cause their separation, and we 
may thus be enabled to obtain some definite information of the 
amount of matter which is necessary to maintain the existence 
of these wandering bodies. 

Though the great eccentricity of the ellipses, which these 
rare bodies describe, and the imperfect manner in which their 
parts are frequently held together, will render it impossible to 
attain great precision in calculating from this principle the 
lowest limit which can be assigned to their mass, we are 
warranted in rejecting the estimate of Babinet, who announces, 
as the result of his investigations, that a comet as large as the 
earth cannot contain as much as thirty tuns of matter. The 
attractive power of so small a mass could not resist the dis- 
membering action of the sun, unless it were separated from 
him by a distance of 2,000,000,000,000 of miles. A ball of 
iron, six feet in diameter, would weigh about thirty tuns, but 
it would be easy to show that if such a small sphere moved in 
the earth's orbit, the solar attraction would render it incapable 
of confining around it an atmosphere even one thousand feet 
high. The result which Babinet has obtained may, therefore, 
be pronounced several million times too small ; but we must 
condemn an opposite error of many astronomers, who as- 



MASS AND DENSITY OF COMETS. 69 

cribe to comets an attractive force sufficiently great to affect 
the condition of their tails, and to collect the matter which 
has been driven into space by the fierce conflict with the 
solar beams. 

In the preceding chapter, I have shown that M. Babinet's 
results are vitiated by a serious error, in consequence of the 
influence of cometary matter on the solar rays passing through 
it. Had his investigations been sufficient to establish the ex- 
traordinary rarity which he assigns to the nebulous matter, it 
may be regarded as an evidence that a dense nucleus of solid 
or liquid matter must be inclosed within it. Though the pres- 
ence of such a nucleus has been proved only in a few cases, 
yet nearly all of these rare bodies give indirect indications of 
the effects which it might be expected to produce. In respect 
to bulk, the dense materials of the nucleus must be many 
million times less than the nebulous mass that surrounds it. 
At the distance of 95,000,000 miles from the sun, a volume of 
this nebulous matter, thirty times the size of the earth, might 
be held together by the attraction of a globe of granite, one 
hundred miles in diameter, and it is probable that the greater 
number of comets have their integrity maintained by the 
attractive influence of an equally insignificant mass. 

M. Babinet concludes, without adequate grounds, that the 
envelope of a comet, in consequence of its great rarity, must be 
insensible to the elastic forces which subsist between the parti- 
cles of gases, and give rise to the expansibility which they 
exhibit. According to his own experiments, air cannot expand 
to more than twenty thousand times its original volume, on the 
removal of the atmospheric pressure ; but this arises from the 
great elasticity of the aerial particles being equipoised by their 
weight. An expansion several million times as great might be 
reasonably expected in a comet's envelope, where gravity is 
very feeble, and the aeriform matter of which it is composed 
must be too rare, in all places, to refract light to any sensible 
extent. Our atmospheric air has its density reduced four per 
cent, by an elevation of one thousand feet above the earth's 
surface ; but in a cometary atmosphere it could not experience 



70 MASS AND DENSITY OF COMETS. 

the same change of density, on being removed one thousand 
miles further from the nucleus. We cannot, therefore, infer 
from the extreme rarity of the nebulous matter of these 
curious bodies, that it consists of gases wholly different from 
those which fall under our observation, and that it is con- 
trolled by forces unlike any operating on our planet. 



COMETARY CATASTROPHES. 71 



COMETARY CATASTROPHES. 



It has been a favorite object with astronomical writers to 
quiet the undue apprehensions which the appearance of these 
wandering bodies has excited in every age; and the calcula- 
tion of Babinet, alluded to in the last chapter, appears to have 
been undertaken with a view of ascertaining how far a collision 
with one of them could affect the condition of our planet. But, 
though much alarmed for the safety of our own terrestrial abode, 
and of the other worlds of our system, we cannot overlook the 
terrific scenes which the less-favored occupants of space are 
occasionally doomed to exhibit. The peculiar arrangement 
which secures for the primary planets a state of comparative 
repose, and many other advantages we enjoy on this earth 
does not seem to be extended to the comets. The great 
eccentricities of their orbits cannot fail to expose them to very 
injurious extremes of heat and cold, while the great extent of 
their atmospheres must render them subject to movements so 
violent as to endanger the stability of the cometary structure. 
Whenever they arrive in very close proximity with the sun, 
they must experience more fatal consequences from the violent 
heat ; and even his unequal attraction on their parts may 
occasionally lead to their dismemberment. 

The observations on comets give indications of the awful 
scenes which a combination of these circumstanees may be ex- 
pected to produce. The changes which were observed in the 
head of Halley's comet in 1835, could only arise from commo- 
tions of the most terrific character. But the terrible strife of 
elements in these bodies has been exhibited on a few occasions 
in far more unmistakeable characters. At the commencement 
of the year 1846, the surprise of astronomers was excited by 
the division of Biela's comet into two parts, which pursue inde- 
pendent paths around the sun. An account of a similar sepa- 



72 COMETARY CATASTROPHES. 

ration is alluded to by Seneca, who does not, however, profess 
to repose any confidence in it himself. Indirect evidence of 
such extraordinary events on other occasions are not wanting, 
and the existence of a double tail which has been sometimes 
observed, both parts being turned from the sun, may be ac- 
counted for by supposing that two comets had been formed 
from a single one by an occurrence of this kind. According 
to Cheseaux, the comet of 1744 appeared at one time with six 
tails ; and if we can rely on his statement, it would seem that 
these six appendages must have emanated from six parts into 
which the cometary mass was divided by the disturbing forces 
which it had to encounter in its dangerous proximity to the 
sun. 

The commotions which the influence of solar heat may 
occasion in the envelope of a comet, we can only hope to under- 
stand by first obtaining an acquaintance with the storms which 
interrupt the tranquillity of our atmosphere. It is in warm 
climates that these aerial movements assume their most dread- 
ful character, and their violence seems to be in some degree 
proportionate to the amount of vapor contained in the air. 
The approach of a tornado in tropical climates is generally in- 
dicated by the appearance of a dark cloud in the distant 
horizon, and the inhabitants of these regions regard this sign 
as the harbinger of more awful scenes, which they must witness 
in a few hours. From more extended observations it appears 
that excessive showers of rain or hail are descending in the 
place where this cloud occurs, while the air is moving to it, 
with furious impetuosity, from all surrounding localities. But 
to render this movement possible, there must evidently be an 
ascent of air at the focus of the storm, or the point to which 
the course of the conflicting winds is directed ; and this ascent 
is proved by many facts, especially those derived from barom- 
etrical observations. 

Several circumstances lead to the conclusion that the fall of 
the heavy rains in these cases must be intimately connected 
with the atmospheric movements, and the connection has been 
explained, in a very satisfactory manner, in Professor Espy's 
theory, or philosophy, of storms. As evaporation is attended 



COMETARY CATASTROPHES. 73 

with cold, so the return of watery vapor to a liquid form must 
occasion an elevation of temperature ; and from the result of ex- 
periments on a small scale, it appears that the amount of heat 
evolved by the return of many thousand tuns of aqueous vapor 
to a liquid condition, must expand the air so much as to pro- 
duce an ascending current in the region where the most heavy 
rain is descending. The colder air around, accordingly, flows 
into this locality, and ascends as it becomes warm, while the 
watery vapor it bears condenses during the ascent, and con- 
tributes its supply of heat for maintaining the great movement. 
This condensation is the necessary result of the cold occasioned 
by the expansion of the ascending air. The low temperature 
arising from this cause is, however, overbalanced by the heating 
influence of the condensing vapor when the air is nearly 
saturated with moisture ; but when it is reduced to a state of 
comparative dryness, an opposite result may be expected. This 
accounts for the fact that the most considerable showers of rain 
commence with an unusual heat, and end with a correspond- 
ing degree of cold. According to Espy, great fires might 
occasion rains and storms, by giving the atmosphere its in- 
cipient movement, and causing the condensation of vapor to 
begin. Among various instances which might be adduced in 
confirmation of this conclusion, we may mention the confla- 
grations of Moscow and Sevastopol, which were accompanied 
with the most violent storms. 

But, though admitting the general accuracy of Mr. Espy's 
views, we cannot overlook the influence of another agent, which 
takes a very decided part in atmospheric phenomena. Like all 
light bodies, the air which surrounds us is very sensitive to 
electrical attraction and repulsion, and the pressure of our 
atmosphere must be somewhat augmented by the high state 
of electricity in its upper stratum. To this elevated region an 
excess of the electrical fluid is confined, in consequence of the 
imperfect conducting power of the air near the ground, until the 
insulation is broken by the condensation of vapor, and the fall 
of rain. As the presence of moisture affords a passage for the 
escape of electricity from the higher atmosphere, the air is 
repelled from the earth, and forms an ascending current. 



74 COMETARY CATASTROPHES. 

During the ascent it is cooled by expansion, and its watery- 
vapor condenses, so that the electrical discharge is continually 
repeated as the partial vacuum is filled by the influx of air 
from surrounding localities. This electrical convection, as it 
is called, Dr. Hare regards as the great motive power of the 
air in all hurricanes. In the present case, we can only regard 
heat and electricity as the two chief agencies concerned in 
atmospheric movements in the manner in which Espy and Hare 
have pointed out, but it would be difficult to determine the 
relative part which must be assigned to their motive powers in 
the disturbance of our aerial ocean. 

From the great violence of hurricanes in our warm climates, 
we may reasonably conclude that they would acquire an in- 
tolerable fury, if our globe occupied the orbit of Venus; while, 
on the contrary, our atmosphere would be reduced to a state 
of almost entire stagnation, if we were removed to the region 
of the asteroids. It must be recollected, however, that the 
movements in the great ocean of air would become violent or 
feeble, as its depth was increased or diminished, and there is 
reason to believe that the atmospheric covering of all large 
planets has its height increased in proportion as they are distant 
from the sun. Such an arrangement, beside securing them 
from the deleterious influence of calms and storms, would have 
an important influence in equalizing the sun's calorific influence 
on near and distant worlds, so that the climates of all may be 
compatible with the purposes of life. But the comets cannot 
be expected to have their constitution so well adapted to the 
regions which they occupy, and accordingly they must suffer 
much from the movements of their nebulous matter, as they 
feel the effects of an intense solar heat, while passing through 
the central domain of our system. If water, or any other 
volatile fluid, formed a considerable part of the dense matter 
which is required to hold together the rare cometary gases, 
much vapor would be generated on the approach of the comet 
to the center of our system. Wherever this vapor began to 
condense, the liberation of heat, and the discharge of electricity 
to the central nucleus, would give rise to a great current in the 
rare fluid, and determine the focus of a storm. Owing to its 



COMETARY CATASTROPHES. 75 

vast height, the greater part of the nebulous appendage would 
participate in the movement, and give it a degree of impetu- 
osity which the feeble attractive power of the nucleus could 
scarcely control. If ascending currents of air on our own 
planet often prevent condensed vapor from falling, until it forms 
large drops of rain, hail-stones of a considerable size, or water- 
spouts in some cases, the vapor returning to a liquid state in the 
atmosphere of a comet, where gravity is many thousand times 
more feeble, might be sustained by similar ascending currents, 
until it had collected into bodies as large as lakes or seas. — 
Even large collections of the fluid evaporated from the central 
nucleus may be sometimes driven, in this manner, beyond the 
sphere of the comet's effective attraction, and may separate for- 
ever from the central mass, taking away, by its attraction, much 
of the attenuated matter composing the envelope. 

Such is the manner in which I accounted for the division of 
Bicla's comet, some time since, and I adduced, in confirmation 
of my views, a remarkable fact, for which no explanation ap- 
pears to have been hitherto offered. About five or six weeks be- 
fore reaching the perihelion of its orbit, the principal part into 
which this body divided was about eight times as large as its com- 
panion, and four times as bright ; but in approaching the sun, 
the disproportion between them gradually disappeared, until, at 
last, the companion acquired the superiority in size and brillian- 
cy, which, however, it retained for only three or four days. — 
On retiring from the sun, they gradually assumed the same rel- 
ative proportions which they possessed on the first appearance. 
On the last return in 1852, the same phenomenon was observed; 
the parts being almost equally large and bright when nearest 
to the sun, but appearing very unequal at greater distances 
from this body. These singular results can only be accounted 
for by supposing that the two parts differed much in their ca- 
pabilities of affording materials for evaporation, and it is pre- 
cisely what would occur if one were fluid, and the other com- 
posed almost exclusively of solid matter. On approaching the 
sun, the nebulous appendage of the first would swell by the 
introduction of vapor, while the small amount of vapor con- 
tained in the other, would be only rendered invisible by solar 



76 COMETARY CATASTROPHES. 

heat. We cannot account for the unequal change in their 
appearance by supposing an actual transfer of matter from 
one body to the other, as they were 150,000 miles apart in 
1846, and were separated by ten times that distance in 1852. 

Although modern observation furnishes no other instance in 
which the indirect action of solar heat caused the volatile mat- 
ter of a comet to boil over the boundary prescribed by its at- 
traction, yet indications of the violent commotions which these 
bodies experience in the vicinity of the sun, are not wanting. 
The luminous sectors or fan-like flames, which issued from 
the head of Halley's comet in the year 1835, have afforded a 
subject for much speculation, and their appearance seems to 
correspond to what might be expected from the occurrence of 
cometary storms. The emanations displayed an extraordina- 
ry brightness, which must be ascribed, not only to the reflec- 
tion of the solar beams, but also to the discharge of electricity 
from the condensing vapor. 

But the action of the solar heat on the nebulous mass of 
comets, does not constitute the sole danger to which these 
wandering bodies are exposed. The unequal attraction of the 
sun must often be fatal to the stability even of the more dense 
part of their nucleii, whenever they come into very close prox- 
imity with him. From what has been shown, in the fifth 
chapter, in regard to the stability of satellites, in small orbits, 
it will readily appear that if the lunar orb were made to revolve 
around the sun, within two hundred thousand miles of his sur- 
face, it would scarcely have the power of holding its parts 
together, in consequence of the great disturbing influence of 
solar gravity. The planet Jupiter, if placed in similar circum- 
stances, would, in consequence of its low density, be wholly 
incapable of preserving its planetary form. 

The great comet of 1680 approached within fifty thousand 
leagues of the sun's surface, while the comet of 1843 swept 
still closer to the solar sphere. Had the nucleus of either of these 
bodies been equal to the moon in size and density, it could 
scarcely have avoided being ruptured by solar attraction in its 
perihelion passage. While the dismemberment of a comet is 
regarded as a matter of astonishment, it would be far more 



COMET ART CATASTROPHES. 77 

surprising that they should escape such a fate in so dangerous 
a proximity to the sun. It would be impossible to ascertain 
precisely what changes, or casualties, the comets of 1680 and 
1843 experienced in their rapid journey through the central 
part of the solar system-; but if it could be made to appear that 
their nucleii preserved the integrity of their structure, it may be 
regarded as an evidence that they must have consisted of solid 
matter somewhat more dense than the rocks composing the 
earth's crust, though the nebulous mass which surrounds them is 
several thousand times more attenuated than the air we breathe. 
Even in cases where there was no general dilapidation of the com- 
etary structure, the fierce heat of the sun must frequently have 
call'ed forth the explosive force of confined gases, detaching many 
fragments from the mass, and compelling them to seek separate 
routes through the planetary domain. The small size of these 
bodies would not prevent them from becoming independent 
members of our solar system, and performing numerous revo- 
lutions around the great central sphere, but some of them 
could not avoid striking the large planets, and we thus arrive 
at a very plausible explanation of the origin of meteoric stones. 
The fall of these singular bodies to the earth has long been 
regarded as a matter of surprise, but it is only in comparatively 
recent times that they have become objects of scientific interest. 
It is now universally admitted that they come from the regions 
of celestial space, though the precise locality which furnishes 
such vast numbers of them is still a subject of speculation. 
The opinion that they are projected from lunar volcanoes, 
though first advocated by Laplace, is now generally abandoned. 
Beside the objections derived from their composition, and the 
absence of any visible volcanic eruptions in the moon, Hum- 
boldt deduces another, from a consideration of their velocities, 
which, after making due allowance for the effects of terrestrial 
gravity, he finds far greater than any volcanic force could com- 
municate. It has been urged that they belong to the numer- 
ous family of asteroids, of which such great numbers have 
been recently discovered, between Mars and Jupiter, and that 
they are to be regarded as the smaller fragments of an ex- 
ploded member of our system. But it may be remarked, that 



78 COMET ARY CATASTROPHES. 

their similarity in composition and external character is 
greater than we could expect to find in the different parts of a 
shattered world. Chladni first suggested the idea that me- 
teoric bodies are a class of diminutive satellites to the sun, 
and this idea has been embraced by Humboldt to the exclusion 
of the lunar hypothesis. To use the words of this great 
writer : " The view of the original existence of small planetary 
masses in space, is more simple, and, at the same time, more 
analagous with those entertained concerning the formation of 
other portions of the solar system." 

With due deference to the judgment of this illustrious phi- 
losopher, I think that a slight modification in the views which 
he adopts might seem admissible. It has been estimated by 
Chladni, that about seven hundred meteorites strike our globe 
every year. The planets Jupiter and Saturn, from their great 
size and attractive power, must annually receive several millions 
of these visitants from space. While such vast numbers an- 
nually drop from the stage of planetary existence, it cannot be 
considered unphilosophical to suppose that there is some means 
for replenishing the waste, and the present researches on the 
physical constitution of comets, point to an adequate cause. 
If the nucleus of the comet of 1843, or that of 1680, con- 
sisted of any considerable proportion of solid matter, they 
could not pass in so dangerous a proximity to the great central 
fire without scattering many of their fragments into space, and 
populating the solar regions with vast numbers of these cos- 
mical masses. 

From the anomalous composition of meteoric stones, we may 
learn much respecting the nature of the causes concerned in 
giving them birth, and the adventurous scenes they must have 
experienced when starting forth in their career of cosmical 
existence. Launched forth into space, in such close prox- 
imity to the sun, and compelled to return periodically to the 
same region, they must feel a degree of heat sufficient to dis- 
sipate most of the volatile materials associated with them, so 
that their composition would ultimately exhibit a preponder- 
ance of those elements which are best calculated to withstand 
the dissipating influences of excessively high temperatures. 



COMETARY CATASTROPHES. 79 

Now, of all substances which occur in nature, in any abun- 
dance, iron and nickel are volatilized with the most difficulty 
when uncombined with other elements, and it is the distin- 
guishing feature of meteoric masses that they contain these 
two metals in excess, and often in a state of great purity. In 
some cases ninety-six per cent, of the entire mass consists of 
iron. Another indication of the effects of the great central 
fire of our system, is presented in the crust which incloses these 
bodies, and which could not be produced by the greatest heat 
employed in our porcelain ovens. 

But the most decisive evidence of the effects of solar influ- 
ence on these mysterious bodies, is to be derived from the 
metallic condition in which the nickel and iron occurs in them. 
This striking peculiarity has given rise to the opinion that they 
must have originated in regions where water is absent, and 
where they are preserved from the action of oxygen and other 
active elements. But it is certainly difficult to imagine that 
any large spheres, where there are such restraints to a chem- 
ical action, could furnish explosive forces sufficiently powerful 
to project any bodies into space. Terrestrial volcanoes should 
have their propulsive power increased more than a thousand 
fold, to project a stone beyond the sphere of the earth's 
attraction. 

The mysterious condition in which the metals appear in 
meteorites, will cease to be surprising when we consider the 
instances which chemistry furnishes of the sun's influence in 
deoxidating bodies, and counteracting the work of chemical 
affinity. The part which the solar rays take in decomposing 
carbonic acid, and preserving the balance of organic nature 
is well known, and the progress of chemical discovery has 
served to reveal other instances of its powers of decomposi- 
tion. But it is in the central department of our system that 
the solar beams must possess the greatest efficiency for over- 
powering the force of affinity, and the reduction of metals to 
a pure state, must be the inevitable result of their decompos- 
ing action. Such comets as plunge into a region where such 
great chemical agencies are ready to operate upon them, must 
have their metallic ingredients disunited from other elements, 



80 CCMETAEY CATASTROPHES. 

and the meteoric masses which they send into space must feel 
a similar change from the influences to which they are sub- 
jected in passing the perihelion of their orbits. 

However difficult it may appear to observe such chemical 
operations from a distance of several million of miles, there 
are still some indications of their occurrence in the more dense 
parts of cometary bodies moving in the neighborhood of the 
sun. To their influence we may ascribe the change in the 
dimensions of the nebulous mass, for the heads of all comets 
grow small as they approach the sun, and enlarge as they are 
retiring from him. Indeed, the feeble attractive power of the 
nucleus must be incapable of retaining so extensive an atmos- 
phere, in passing the perihelion of its orbit, and the subsequent 
augmentation of size seems to take place at the expense of the 
gaseous elements which the solar beams liberate from the 
solid or liquid parts of the nucleus. The violent heat might 
give comets an extensive envelope of vapor, but this would 
disappear, by cold on the verge of the system, before their 
subsequent return, so that permanent accessions to the atmos- 
pheres of these bodies could only arise from the chemical 
powers of the sun's light, which acts on the nucleus, and re- 
stores its confined gases to aerial freedom. 

In his sublime delineation of the several members of the 
solar system, Humboldt gives meteorites and fireballs a place 
in juxtaposition with comets, and other writers have followed 
his example, in treating both classes of the sun's attendants 
under the same head. The similarity of meteoric and come- 
tary orbits has hitherto furnished the only basis for this classi- 
fication ; but it would seem that the relationship between these 
two great divisions of the solar family, is much closer than has 
been supposed. It is fortunate for the extension of human 
knowledge that so many of these fragments find their way to 
our planet, and afford us a specimen of extra-terrestrial matter. 
As these fragments are infinitely more numerous than comets, 
the probability of their striking the planets, will be great in the 
same proportion, and a recent writer asserts, with some degree 
of propriety, that of both classes of celestial wanderers, me- 
teoric stones are the more dangerous to the human race. But 



COMETARY CATASTROPHES. 81 

if, as I have shown reasons for believing, these fragments 
are hurled from comets, on sweeping very close to the sun, 
our views respecting the wanderers of space must be modi- 
fied, and we must regard the cometary warfare, from which 
such dangers have been apprehended, as only carried on by a 
constant bombardment, which can never inflict any serious 
injury on our planet. 
6 



82 PHENOMENA ATTENDING 



PHENOMENA ATTENDING THE EALL OF METEORS. 



The rank which meteoric stones, fireballs, and shooting-stars 
hold among the numerous attendants of the sun, must render 
their visits to the earth an object of deep interest to the student 
of nature. But, apart from any knowledge we possess of the 
previous history of meteoric masses, their composition serves to 
distinguish them from every terrestrial substance that comes 
under our notice. In addition to this, their descent to the earth 
is rendered so conspicuous by the emission of light, and the 
noise with which it is accompanied, that it rarely fails to attract 
the attention which its importance demands. Of the occasional 
precipitation of stones from the region of the clouds, many 
instances have been recorded by historians, but it is on the 
observations of modern times that we must place our chief 
reliance for the facts required for scientific investigation. 

Among the remarkable occurrences of this character ob- 
served during the present century, we may refer to the meteoric 
phenomenon, which was witnessed at Weston, Connecticut, on 
the 14th of December, 1807. About six in the morning, a 
large globe of fire was seen pursuing its course, with con- 
siderable rapidity, along the sky, until it disappeared with a 
loud explosion. In a short time afterward distinct sounds 
were heard, indicating, as was supposed, the precipitation of the 
fragments to the ground. The united weight of all these frag- 
ments did not exceed four hundred pounds, though the diame- 
ter of the fireball from which they were thrown, was estimated 
at five hundred feet. On the 26th of April, 1803, a more con- 
spicuous meteor appeared at L'Aigle, in France, and it called 
forth the astonishment of the scientific men of that nation. A 
large fireball was observed moving rapidly from southeast to 
northwest, while the sky was quite clear, and it finally broke, 
with a terrific noise, into about three thousand fragments, 



THE FALL OF METEORS. 83 

which were scattered over a space of many square miles. A 
fall of meteorites, on an equally extensive scale, took place at 
Crema, in Italy, in the year 1511, and they caused the death 
of a man, besides leaving many other marks of their violence 
behind them. 

Every year furnishes fresh instances of the descent of these 
bodies from the sky ; and a number of observations seem to 
show that, on an average, about one fall annually occurs 
throughout the extent of territory composing the British Isles 
and France. As this comprises about one seven-hundredth 
part of the earth's surface, and as there is no reason that the 
celestial visitors should select particular localities to alight upon, 
Chladni calculates that about seven hundred of them annually 
arrive at our planet. Meteorites have been found in several 
places where no record of their fall has been preserved, and, 
from the presence of nickeliferous iron and their other peculiar 
characters, there could be no doubt entertained that they were 
strangers on our globe. 

The velocity with which these bodies enter our atmosphere 
appears to be often equal, and even superior to that of the 
earth, in its annual movement around the sun. This has been, 
indeed, inferred from observations made on the shooting-stars, 
but, in many cases, independent evidence has been obtained 
of the immense speed of meteoric stones in their approach to 
our globe. The tremendous resistance of the air soon, how- 
ever, deprives them of their rapid motion, and their velocity in 
striking the ground is little superior to that of a cannon-ball. 
Their height above the earth's surface can only be ascertained, 
with any degree of accuracy, from a comparison of the observa- 
tions at the several localities from which they are visible. The 
interval which elapses between the sound and the extinction of 
their light cannot be regarded as furnishing a correct basis for 
calculating the distance of these bodies, and the estimates 
derived in this manner must be rejected as unworthy of confi- 
dence. 

Solid masses moving through the air, receive from, and im- 
part to it a degree of pressure nearly proportional to the square 
of their velocity. Had our aerial ocean been as dense in its 



84 PHENOMENA ATTENDING 

upper stratum as it is near the earth's surface, a meteoric stone 
plunging into it with one hundred times the speed of a cannon- 
ball would receive a pressure of about eighty tuns to the square 
inch, and this is far too great for the strongest materials to 
endure. Accordingly, these adventurous bodies are generally 
broken by their encounter with the atmosphere, especially 
when they descend to the ground in a course almost coincident 
with a vertical line: for, in these cases, they enter the dense 
air before sustaining any great loss of their motion. When, 
however, their course approximates to a horizontal direction, 
and their velocity is gradually reduced by a transit through 
the higher atmosphere, they are more likely to escape a 
rupture, and to make less noise in reaching the ground. 

It is to be regretted that the consequences which these 
bodies must experience from encountering the air with their 
enormous velocities are so little understood, and that most 
persons . are so much disposed to regard the rending of a 
meteorite into fragments as analogous to the bursting of a shell 
by gunpowder. The impression generally prevails, that the 
rupture of a meteorite, and the crash with which it is accom- 
panied, are due to the agency of some inflammable or explo- 
sive compounds, which have their energies aroused into a state 
of activity by the presence of the atmosphere. But there is 
no ground for introducing this gratuitous hypothesis, and the 
fact that such masses often cause the most terrific crash in 
descending through the air, without undergoing any rupture, 
may be considered a sufficient reason for rejecting it. Neither 
is it correct to regard the greatest noise which is heard on such 
occasions as accompanying the rupture, for the sounds which 
are subsequently emitted, though of inferior intensity, must be 
generally more audible from the shorter distance which they 
travel, and the more effective manner in which they are con- 
ducted by a more dense stratum of air. 

The most remarkable phenomenon which characterizes the 
appearance of meteorites is the great light which accompanies 
their visits to the earth. This, in some instances, has been 
displayed to such an extraordinary degree that the bodies have 
appeared as luminous balls of fire, rivaling the sun in bril- 



THE FALL OF METEORS. 85 

liancy during their brief career through the firmament. Often, 
however, the emission of light takes plaee on a much smaller 
scale, and in some cases it is almost imperceptible. Many 
writers regard fireballs and meteoric stones as two different 
elasses of objects. But a more complete examination of ob- 
served facts shows that there is an infinite number of grada- 
tions in the scale on which meteoric light is developed, and 
that no line of demarkation can be drawn between luminous 
and non-luminous meteorites. Humboldt includes these bodies 
and even shooting- stars in the same class, and the propriety of 
this classification will appear more obvious from the researches 
on meteoric light given in the following pages. 

At the first view it would seem natural to ascribe the lumi- 
nous appearance of meteors to the pressure they impart to the 
atmospheric air. If, however, their light originated from this 
cause, it would be displayed on the most conspicuous scale 
when they descend in an almost vertical direction, and when 
their tremendous compression of the air is marked by the loud- 
est report. Yet, in such cases, little or no light is to be ob- 
served to accompany their fall. On the contrary, the most 
brilliant meteors move nearly parallel to the horizon of those 
places where they are visible; and it appears that, as a general 
rule, all meteoric bodies show less power of developing light 
in proportion as their paths deviate from a horizontal direction. 
The celebrated meteor seen at Bononia, in 1670, was equally 
distinguished for its extraordinary brilliancy, and for the close 
approximation of its path to an exact parallelism with the 
earth's surface. Indeed, Halley. in describing this wonderful 
object, considers it as unaccountable that a body could be pro- 
jected in a direction so nearly horizontal. The same peculiarity 
was to be observed in the path of the great luminous balls 
which swept over England in 1719 and in 1783. These me- 
teors were arnon^ the most brilliant on record — the first having 
been said to have rivaled the sun in illuminating power. 

If the evidence on which we depend for a knowledge of this 
important fact should seem liable to any kind of error or un- 
certainty, we may obtain more satisfactory information on the 
£ubject by a comparison of the extent of the tracks which these 



86 PHENOMENA ATTENDING 

bodies described while visible, for their range must be extensive 
in proportion as they moved close to a horizontal direction. 
Now, the great meteor of 1783 was seen sweeping from the 
northern part of Great Britain to the confines of the Mediter- 
ranean Sea — describing a track which, according to the lowest 
estimate, must have exceeded one thousand miles. Unless the 
course of this body when first observed was horizontal, or 
nearly so, it would be impossible for it to pass over so exten- 
sive a range; and we may form a similar conclusion in regard 
to a conspicuous meteor which Kepler has described as passing 
over a large extent of the German States, in his time. The 
meteor of 1719, of which Halley has left so able a description, 
continued its luminous career over a space of seven or eight 
hundred miles, for it passed over the whole extent of Great 
Britain and a considerable part of the ocean, where its course 
could not be observed. It was only in the southern extremity 
of England, where it was last seen, that any noise accompanied 
its appearance, and this indicated its first entrance into air of 
any appreciable density. In the meteorites seen at Weston, 
Benares, and L'Aigle, as well as others, whose luminous appear- 
ance was exhibited in a moderate degree, there was a consid- 
erable departure from a horizontal course. But, as far as can 
be determined from the records of observation, it would seem 
that scarcely any light accompanies the fall of aerolites, which 
descend in a direction almost perpendicular to the earth's sur- 
face, and make the most terrific noise by their collision with 
the denser air. 

The most extraordinary circumstance connected with lumin- 
ous meteors is the great magnitude of the ball of light which 
they exhibit previous to their arrival at the ground. From a 
trigonometrical measurement it has been estimated that the 
Weston meteorite must have been five hundred feet in diame- 
ter, and if it consisted wholly of solid rock, it must have weighed 
over five millions of tuns. Yet all the stones which were pre- 
cipitated to the ground on this occasion could not furnish even 
one-fourth of a tun of solid matter. The diameter of the me- 
teors or luminous balls of 1719 and 1783, to which allusion 
has been already made, has been estimated at over half a mile, 



THE FALL OF METEORS. 87 

and an equally great magnitude has been assigned to several 
others. Now, a solid globe of such huge dimensions must 
contain several million times more matter than has ever been 
known to have been precipitated to the earth's surface. A be- 
lief in the existence of such gigantic meteors is very general 
among writers on meteorology, but it is supposed that these 
huge masses, instead of coming in collision with our planet, 
only sweep along the verge of the atmosphere, and, after cast- 
ing a few fragments to the earth, continue their course through 
the regions of space. But solid bodies of such vast proportions 
could never be expected to rebound from the atmosphere, and 
it is utterly inconceivable that in all their approaches to our 
planet they should invariably take their way through the stra- 
tum of rarefied air, which is incapable of arresting their pro- 
gress. So narrow is the path by which it would be possible 
for them to retire from so dangerous a proximity with our globe, 
that it must be considered as surprising if even one of every 
thousand which strike our atmosphere should escape destruc- 
tion. The soldier, who would state that he was grazed by a 
thousand bullets without being ever struck directly by a single 
one, could not be considered more extravagant in his assertions 
than many of our writers on meteorology, who teach that so 
many of these cosmical missiles graze the earth's aerial en- 
velope without being in a single instance dashed against its 
surface. 

In my writings, in 1853 and 1854, I pointed out the great 
improbability that meteoric masses are capable of performing- 
such miraculous feats of precision, and I contended that the 
great display of meteoric light is occasioned by the small 
amount of solid matter which arrives at the ground on these 
occasions. During the Washington meeting of the American 
Association for the Advancement of Science, I submitted to 
Professor J. Lawrence Smith my views on the subject, as they 
were set forth in a pamphlet which I published a feAV months 
before. In the paper, or rather the oral dissertation on mete- 
orites, which Professor Smith brought before the scientific con- 
vention, no allusion was made to the enormous magnitude as- 
cribed to these bodies ; but this point was elaborately discussed 



88 PHENOMENA ATTENDING 

in a subsequent essay, which he contributed to the American 
Journal of Science and Art for May, 1855. In this his views 
in regard to the non-existence of the supposed gigantic mete- 
orites are similar to those which I published in 1853 and 1854; 
but he supports them with new evidence, part of which was 
derived from the result of experiments made by himself. He 
found that intensely bright objects, viewed from a great dis- 
tance, show a disk several hundred times greater than their 
size would entitle them to possess, and from this he concludes 
that no reliance can be placed on the results of trigonometrical 
measurements for ascertaining the actual dimensions of meteor- 
ites, or the globes of light which surround them. 

I cannot, however, acquiesce in this conclusion, as it appears 
to sap the foundation of all astronomical knowledge, and to in- 
validate the just claims of mathematics to accuracy in furnish- 
ing information respecting the heavenly bodies. If such opti- 
cal deceptions be inseparable from the appearance of bright 
objects, we must be compelled to fix the diameter of the sun at 
less than one thousand miles. From my own observations, I 
am convinced that the great ball of light seen in Professor 
Smith's experiments was composed of illuminated air, which 
became visible as distance rendered the glare of the bright cen- 
tral point less overpowering to the eye. Of this fact any one 
may convince himself by observing, during a dark night, a very 
luminous object, with conspicuous marks placed around it. On 
increasing the distance of this object from our view, the body 
of light it emits will be shown to swell beyond its previous 
boundaries, and its dependence on aerial reflection will be most 
conclusively demonstrated. As the air, at an elevation of forty 
or fifty miles, is too rare for reflecting light to any sensible ex- 
tent, this cause cannot produce any material enlargement in 
the apparent diameter of meteors, and we may accordingly 
rely on the trigonometrical estimates of the size of the ball of 
light which enshrouds those visitants from external space. 

The greatness of these illuminations cannot be ascribed to 
any mechanical action which meteorites exert on the air, for it 
ceases entirely when this mechanical action becomes most en- 
ergetic, as the body encounters the more dense strata of the 



THE FALL OF METEORS. 89 

atmosphere. But more conclusive evidence against the sup- 
posed effects of aerial action, may be derived from a considera- 
tion of the actual height at which these objects are visible- 
According to the estimate of Dr. Halley, the great meteor of 
1719 was over seventy miles above the earth's surface during 
its career of brilliancy, and a far greater elevation has been 
ascribed to a large meteor which appeared in France in the 
year 1847. Now, at an elevation of sixty miles, the air, if not 
entirely absent, must be over a million times more rare than 
that near the earth's surface, and the effects of mechanical 
action in evolving heat from it, must decline in the same pro- 
portion. A cannon-ball, flying with its ordinary speed close 
to the ground, would have far more cause to be luminous from 
the effects of mechanical action, than a meteoric stone forcing 
its way, one hundred times more rapidly, through the rarefied 
air at an elevation of forty-five miles. Some writers have 
ascribed the development of meteoric light to the inflammation 
of combustible gases, which encompass the cosmical mass ; but 
no appreciable complement of gaseous elements could be con- 
fined around bodies of such feeble attractive power, and cer- 
tainly none could be retained by them during their flight through 
the aerial regions. 

Of the numerous attempts which have been hitherto made to 
account for the luminous appearance of meteors, none appear 
to be satisfactory, nor must it be supposed that the objections 
to them have been magnified through a partiality for the new 
theory, which will be introduced in the next chapter. Indeed, 
the defects of received opinions on this subject have been long 
admitted, and Humboldt has very ably depicted the defective 
condition of our present knowledge on this subject. "If we 
admit," says he, " the existence of particular aerial fluids in 
the inaccessible regions of meteors, falling-stars, and the 
Aurora Borealis, how can we conceive that the whole stratum 
of these fluids does not at once take fire, but that the gaseous 
emanations, like clouds, occupy only limited spaces ? How 
can we suppose an electrical explosion without some vapors 
collected together, capable of containing unequal charges of 
electricity in air, the mean temperature of which is twenty-five 



90 THE FALL OF METEOES. 

degrees below the freezing point, and the rarefaction of which 
is so inconsiderable that the compression of an electric shock 
could scarcely discharge any heat ?" Even more recently, in 
treating on aerolites, in the first volume of his Cosmos, he re- 
gards the origin of their light as still involved in obscurity. 
Meteoric astronomy is, however, slowly emerging from its im- 
perfect condition ; shooting-stars are no longer regarded as 
unworthy attention, and it will be shown, in the next chap- 
ter, that even these insignificant objects furnish the most im- 
portant intelligence, in regard to the great operations and events 
which transpire in the physical universe. 



ORIGIN OF SOLAR AND METEORIC LIGHT. 91 



THE ORIGIN OF SOLAR AND METEORIC LIGHT; 



The mysterious cause which gives meteors their luminous 
appearance claims more prominent attention, on account of the 
insight it affords into the great chemical phenomena of the 
heavens. It is only by a study of the luciferous operations 
which nature displays on our own planet, that we can become 
acquainted with the effective plan devised for the production 
of light on the innumerable suns of our universe. The mere 
appearance of stellar and meteoric bodies is calculated to im- 
press the mind with the idea of a very intimate relation in the 
causes of their brilliancy, nor can their inconceivable disparity 
in size militate against this opinion. The forces which per- 
vade the realms of nature, occasionally display their influence 
on the largest and the smallest scale, and the mysterious 
agency which surrounds falling meteorites with such enormous 
globes of light, might seem well adapted to confer on the sun 
his wonderful illuminating energy. But the present researches 
claim more interest than they might be entitled to receive, if 
they only furnished information of the manner in which our 
own system is warmed and illuminated. As the conditions 
necessary for the existence of self-luminous orbs become 
known, the grandeur of remote systems will appear more im- 
posing, and the stellar universe will become a more fertile field 
for the inquiries of the astronomer. 

Notwithstanding the intelligence which science gives of their 
surprising magnitude, suns and worlds must be regarded as 
very insignificant when compared with the vast extent of their 
celestial abode. So great is the disproportion between the 
dimensions of the heavenly bodies, and the immense distances 
between them, that in most cases it has been found impossible 
to have the relation accurately represented by means of dia- 
grams or orreries. If a true map of the solar system were to 



92 ORIGIN OF SOLAR 

be drawn on a scale of one hundred thousand miles to an inch, 
a circle one mile in diameter would be required to represent 
the orbit of Neptune, while a diameter of three thousand feet, 
fifteen hundred feet, and eighty feet respectively, should be 
given to the circles or ellipses representing the orbits of 
Uranus, of Saturn, and of the earth. But on so large a map, 
the figure of the sun could not exceed nine inches in diameter, 
while that of Jupiter and Saturn would be less than one inch, and 
that of the earth must be a small dot which the head of a pin 
might almost cover. Even such a diagram could not afford a 
correct comparison between the extent of the solar system and 
the size of its members, for the wide space which the latter 
occupy should be (strictly speaking) represented by a globe 
(and not a circle) one mile in diameter. Were it required to 
have a map of the fixed stars drawn on the same scale, we 
should be compelled to separate them, in most cases, by dis- 
tances of eight or ten thousand miles, so that even these great 
bodies, though rivaling our sun in magnitude, can be consid- 
ered as only mere points compared with the vast domain over 
which their controlling influence extends. 

The contemplative mind must regard the immensity of space 
with deeper interest, in consequence of the information which 
modern astronomy gives of its boundless resources. Besides 
serving as the abode of many suns and worlds, it is filled with 
a subtile fluid, several million times more attenuated than the 
air which sustains life on our planet. Of the existence of this 
space-pervading fluid, some proofs have been introduced in 
the, foregoing chapters, and others of a more convincing char- 
acter will be furnished by the present investigation. But the 
evidence derived from the observations on Encke's comet, 
though not the most satisfactory that can be produced, has been 
considered by most astronomers as sufficient to establish the 
fact that such an etherial fluid is diffused through the celestial 
domain. With some, indeed, it is still a favorite opinion that 
illimitable space is an absolute vacuum, as they feel reluctant 
to entertain the idea that any such impediments can oppose the 
movements of the planets and prevent the eternity of their 
existence. But tidal action must be attended with like conse- 



AND METEORIC LIGHT. 93 

quences to the motion and the duration of the members of the 
solar system, as has been shown in a previous chapter ; and it 
will appear, on a more extended view of the subject, that the 
slight influence of this fluid in shortening the immeasurable 
age of worlds, is overbalanced by the important office it per- 
forms in the sublime economy of creation. 

It is the action of chemical affinity between different kinds of 
matter that contributes most to maintain the great operations 
which are carried on around us ; and, supposing the same 
power to prevail between the elements of the space-pervading 
ether, it will be easy to trace the consequence to which it must 
lead. The fluid is so much rarefied in the interplanetary do- 
main that no chemical changes can take place between its 
elements, except where it is collected around the largest spheres 
and compressed by their powerful attraction. In obedience to 
the law of gravity, which exerts a universal control over all 
matter, atmospheres of the etherial fluid are collected around 
the earth and the other large planets, but they are not suffi- 
ciently dense for chemical action, except in cases where they 
receive an additional pressure from meteoric stones sweeping 
through them with furious rapidity. When these cosmical 
bodies, on falling to the earth's surface, move in a direction 
almost horizontal, they take a longer course through the verge 
of the atmosphere, and the etherial medium is stimulated to 
chemical activity by the pressure not only from the meteoric 
mass itself, but also from the particles of air which it drives 
in every direction from its passage. As such a chemical ac- 
tion must be attended with a development of heat and light, it is 
not surprising that meteorites are luminous before reaching the 
confines of the air, and that their brilliancy is exhibited on a 
gigantic scale when their paths are almost parallel to the hori- 
zon. But, on the surface of the sun, a more powerful at- 
tractive force not only collects a more immense atmosphere of 
this fluid, but also gives it the degree of compression neces- 
sary for a continual chemical action and an unfailing develop- 
ment of light, while the realms of space furnish constant 
supplies of luciferous matter, and maintain, in steady activity, 
the great ocean of flame which heats and illuminates so many 
distant worTds. 



94 ORIGIN OF SOLAR 

That the light of the sun is produced in his atmosphere, was 
long maintained on insufficient grounds, but was at length es- 
tablished in a very satisfactory manner by the observations of 
Arago. The invention of the polariscope furnished this as- 
tronomer with the means of distinguishing gaseous bodies by 
the kind of light which they emit, even when viewed from 
the greatest distances. If an incandescent solid or liquid 
body be observed through this instrument, it will exhibit two 
images having complementary colors, but when a burning gas 
is submitted to a like observation, both images appear white. 
These different appearances must be ascribed to the fact that, 
in a state of incandescence, solids and liquids emit polarized 
light, while the light from gases is wholly unpolarized or 
natural. Arago, therefore, infers that if the sun's rays ema- 
nated from solid or liquid matter, they would be partially 
polarized, and their characters would be revealed by the ap- 
pearance of two differently colored images when the solar disk 
was viewed with the polariscope. Near the margin the obser- 
vations would be attended with the most decided results, but 
near the central part they fail to give any information of the 
nature of the illuminating materials. On a number of trials 
it has been found that the light from every part of the sun's 
disk is devoid of the slightest trace of polarization, and Arago 
has been accordingly led to the inevitable conclusion that it 
cannot proceed from solid or liquid materials, but must be en- 
tirely produced in the aeriform envelope or photosphere by 
which he is covered. 

As a considerable amount of refraction partially polarizes 
light, the solar beams could not exhibit their present charac- 
ters in the polariscope if they rose from the more dense gases 
very close to his surface, and it appears, from other observa- 
tions, that the lower part of the sun's atmosphere is either 
totally dark, or extremely deficient in illuminating power. 
"According to the present condition of our astronomical 
knowledge," says Arago, ** the sun is composed: 1st. Of a 
great central sphere which is dark ; 2d. Of a vast stratum 
of clouds suspended at a certain distance from the central 
body which it surrounds on all sides; 3d. Of a photosphere, 



AND METEORIC LIGHT. 95 

or, in other words, a luminous sphere inclosing the cloudy- 
stratum which, in its turn, envelopes the dark nucleus." It 
must be interesting to find that the dark and luminous atmos- 
pheric appendages of the sun have their representatives on our 
planets. Meteoric stones, fireballs, and shooting-stars are only- 
luminous at or beyond the boundary of our aerial atmosphere, 
and cease to be so on their entrance into the dense air. It is 
also in the extra-aerial regions that the Aurora Borealis seems 
to originate. Of the extraordinary illuminating power of 
the fluid which burns around shooting-stars, we may be con- 
vinced from the vast amount of light which these objects emit 
compared with their diminutive size. Although some ob- 
servers, judging from their luminosity, have ascribed to them 
a diameter of from eighty to one hundred and twenty feet, yet 
from the manner in which so many myriads of them have been 
lost in the atmosphere, during the great meteoric showers of 
1799 and 1833, we cannot assign to them a higher rank than 
hailstones, or drops of rain, so far as actual magnitude is con- 
cerned. Thus the most satisfactory test that we can obtain 
serves to prove the intimate relation which subsists between the 
luciferous appendage of the sun and that belonging to our 
planet. It is also highly probable that the dark or cloudy 
stratum of the solar envelope does not differ much in composi- 
tion from the aeriform fluids that float in our atmosphere. 

All known facts and principles of modern chemistry discoun- 
tenance the idea that solar matter can become luminous with- 
out changing its chemical characters, and accordingly fresh 
supplies of light-yrelding materials are necessary to sustain the 
perpetual luminary of our system. The source of these mate- 
rials, and the manner in which they are supplied in such unfail- 
ing abundance, have long been a subject of speculation with 
astronomers. Many regard the solar mass as a great store of 
combustible matter, or a reservoir of inflammable gases, which 
are continually issuing from his interior regions and burning on 
his surface. But every one must be aware that fire has no 
power of governing its fury when combustible materials are 
placed within its reach ; and, were the fuel derived from the 
sun himself, it would be always supplied in the greatest abund- 



96 ORIGIN OF SOLAR 

ance when the ocean of flame was most violent, and withheld 
when the intense heat began to decline. We could hardly 
hope for success in heating our smallest apartments, if we 
adopted the crude plan which speculative astronomy has de- 
vised for diffusing heat and light through the immense abodes 
of creation. 

Other considerations show that the sun himself cannot be 
supposed to furnish the immense quantity of luciferous matter 
which unceasingly burns around him. According to the re- 
searches of Pouillet, the amount of heat which flows from the 
vast orb could be scarcely produced by the hourly combustion 
of a layer of the densest coal, ten feet thick, and extending 
over his entire surface. Had the sun's matter been expended 
at this rate, the loss would be discernible in a few years, either 
from a reduction in his diameter, or from a change in the plan- 
etary motions in consequence of the decline of his attractive 
power. 

But, if the production of light on the sun depends on the 
combustion of his matter, it must involve a far more rapid 
change in his magnitude. Regarding the light of the full moon 
as equal to that of a common candle eight feet distant, and 
adopting Dr. Wallaston's comparative estimate of solar and 
lunar brilliancy, the sun must afford us the same degree of 
illumination which we might receive from one million of can- 
dles, placed at a distance of three yards. Now, as light dimin- 
ishes in power, in proportion as the square of the distance from 
its source increases, over 3,000,000,000,000,000,000,000,000,- 
000 candles would be required to produce the same effect at 
the distance of 95,000,000 miles, and the quantity of light 
which flows from the solar orb could be scarcely produced by 
the daily consumption of 500 globes of tallow, each equal to 
the earth in size. Had the sun been wholly composed of such 
combustible matter as we use for our various purposes, it would 
be burnt up before a few years in maintaining his wonderful 
brilliancy, and it would require a very high atmosphere even of 
pure oxygen to support so great a conflagration, for a single 
day. 

It is only by the exhaustless supplies of ether from illimit- 



AND METEORIC LIGHT. 07 

able space that so extensive an illumination could be kept up 
for a long period, and the luciferous action is restrained within 
proper limits by the laws which govern the chemical changes 
of the setherial fluid. As a certain density or degree of com- 
pression is always required for the play of affinity between its 
elements, it always burns at the same distance from the center 
of the solar orb, and the great ocean of flame is ever main- 
tained at the same elevation. If the flame had been fed 
by gases emitted from the body of the sun, fluctuations in its 
level would be inevitable, for we could not ascribe any degree 
of uniformity to the emission or eruption of gases from his in- 
ternal regions. The invariability of the boundary of solar light 
will show more clearly the agency concerned in producing it, if 
we admit, on the authority of Arago, that the photosphere of 
the sun is surrounded by another envelope, from which no light 
is emitted. From the well-known property of diffusibility 
which prevails in all elastic fluids, it is absurd to suppose that 
different gases composing the solar atmosphere could separate 
and establish between themselves a boundary as well defined 
and unchangeable as that exhibited by the solar disk. We 
must, therefore, ascribe the want of luminosity in the sun's ex- 
ternal covering, not to any defect in its chemical properties, but 
merely to an absence of the powerful compression which it 
might receive at a greater proximity to his surface. 

The same law which requires the space-pervading aather to 
possess a certain density before it can undergo chemical action, 
will cause it to select the largest spheres as the theaters of its 
brilliancy, and, accordingly, it is ever ready to display its powers 
of illumination on bodies whose attraction is sufficiently great 
to give it an adequate degree of compression. The self-lumin- 
ous character of suns must therefore be ascribed, not to their 
peculiar chemical constitution, but to the immensity of their 
masses, and to the superior energy of the attractive force which 
is felt around their surfaces. For this reason, none of the pri- 
mary or secondary planets belonging to our system partake of 
the sun's capability of developing perpetual light. This was a 
source of great astonishment to Sir Isaac Newton, though in 
his days the known planetary bodies were not more than one- 
7 



98 ORIGIN OF SOLAR 

fourth as numerous as they are at the present time. In his 
opinion, no reason could be assigned why the matter should 
divide itself into two parts — -the luminous portion congregating 
into one mass and forming a sun, while the residue was con- 
verted into a great number of comparatively small spheres. 
But there is no necessity for this unnatural division of mat- 
ter, since, even if the sun were identical in composition with 
his attendants, yet, in consequence of the great superiority of 
his attraction, his surface would necessarily become the fo- 
cus at which the aether of space must display its luciferous 
properties. 

|gfc In other parts of the wide celestial domain, this setherial me- 
dium observes the same rule in selecting large spheres as the 
theaters of its chemical activity; and therefore, in respect to the 
quantity of matter which they contain, the fixed stars may be 
regarded as holding nearly the same rank as the great luminary 
of our own system. This conclusion, in regard to the size of 
the setherial bodies, accords with the views generally held by 
astronomers. A systematic course of observation, continued 
for several years, impressed Sir William Herschel with the be- 
lief that a certain degree of equality existed between the stars, 
and he was led to regard their apparent magnitudes as entirely 
depending on their distances. From this hypothesis he arrived 
at his important results in regard to the form and the extent of 
the universe; and, indeed, the greater part of our present 
knowledge of the stellar bodies is based on the supposed fact 
of their equality. This fact is, indeed, admitted by several as- 
tronomers, but it is questioned by a few, and the author of the 
Plurality of Worlds regards many of the stars as mere dots or 
lumps of light. It is only by a knowledge of the law which 
governs the luciferous processes on these distant objects that 
we can gain any reliable information respecting the relative 
size of the celestial objects, and the present investigations will 
enable us not only to decide between conflicting opinions on the 
subject, but also to establish siderial astronomy on a more secure 
foundation. 

The theory on solar light which has been introduced in the 
present chapter, and of which additional proof will be presented 



AND METEORIC LIGHT. 99 

In the two succeeding ones, does not necessarily lead to the 
conclusion that all self-luminous bodies must have an absolute 
equality in point of magnitude. Such an equality might, in- 
deed, be expected, if all were composed of equally dense mat- 
ter; but the difference that must exist in the density of their 
component paris will leave much room for variety in the dimen- 
sions which each must possess to serve as perpetual luminaries. 
Our sun would cease to be luminous if his matter expanded so 
much as to make his density as low as that of Jupiter or Sa- 
turn, for in this case his surface would extend beyond the range 
within which his light is now developed. If, on the contrary, he 
were as dense as the earth, he would require a diameter of only 
400,000 miles to maintain the incessant manufacture of heat and 
light around him, and a diameter of about 200,000 miles would 
be sufficient for the purpose, if his materials had the specific 
gravity of platinum. Supposing this to be the greatest density 
a celestial body can possess, we are led to conclude that the 
smallest member of the stellar host must have a diameter over 
twenty-five times as great as the earth, and must contain about 
one-sixth of the quantity of matter composing our own sun. 
Accordingly, though the fixed stars do not conform to the law 
of strict equality, we have reasons for believing that their masses 
exhibit a far less difference than we might be led to expect from 
an acquaintance with the several members of the solar system. 
The weight of a remote sun might be determined from its 
action on a planet, but unfortunately the latter bodies are hid- 
den from our view, except in the few cases in which they are 
self-luminous. Observations on the physically double stars, 
whose distance can be ascertained, may supply information on 
this point, but it is only in one or two instances that satisfactory 
results have been ascertained. The two stars composing * 
Centaur are separated by a distance seventeen times as great 
as the radius of the earth's orbits, and both revolve in seventy- 
seven years around a common center of gravity. Now, if a 
planet had so wide an orbit in our own system, it would com- 
plete its revolution in seventy years, and it may therefore be 
inferred that the attraction of the solar mass which is capable 
ef controlling so rapid a movement is about twenty-one per 



100 ORIGIN OF SOLAR 

cent, greater than that of the two distant suns which form this' 
double star. But, supposing that both bodies were as dense 
as the earth, their self-luminous character would present no 
inconsistency with the present theory of solar light. It has 
been supposed that the double star, known to astronomers as 
sixty-one Cygni, has been a still less mass, but more recent 
observations show that little confidence is to be placed in the 
result which has led to this conclusion. 

Although we may thus obtain a satisfactory basis for an 
opinion respecting the smallest size of suns, we cannot speak 
so decidedly respecting the greatest dimensions which these 
bodies may attain. It is impossible to determine how far the 
fierce heat which prevails around such mighty spheres may 
cause a waste of their materials by imparting extreme rarity 
or violent explosive power. The most terrific commotions oc- 
casionally seem to prevail on our sun's surface. Bright streaks 
of light are visible in many parts of his disk, and, during 
eclipses, columns of flame have been observed to rise many 
thousand miles beyond the regular boundary of his light. Our 
acquaintance with meteoric phenomena would naturally lead us 
to infer that showers of rocks, hurled from his internal regions, 
and compelled to sweep through his luciferous atmosphere^ 
would exhibit the observed appearances. 

It has been generally supposed that the dark spots on the 
sun are caused by the emission of gases from his surface, but 
the observations of Peters seem unfavorable to this opinion- 
After a careful examination of the spots for many years, this 
astronomer has recently announced the important fact that they 
do not always occupy the same position on the sun's surface, 
but continually move in the direction of his equator. It will, 
therefore, be more correct to regard them as solar storms, and 
the cause of these commotions, together with the periodicity 
of their occurrence, will be discussed in the next chapter. 
But the change discovered by Peters in the heliocentric latitude 
of the spots deserves a special consideration in this place, as it 
evidently indicates a general movement of the sun's atmosphere 
from his poles to his equator. Such a movement could only 
arise from a continual influx of the space-pervading s&ther to- 



AND METEORIC LIGHT. 101 

ihe poles of the great sphere, while it is expelled from his equa- 
torial reoions by centrifugal force. The sun's rotation is, there- 
fore, partially instrumental in renewing his photosphere with 
supplies of luciferous matter from space, while the same end 
is promoted, in perhaps a still greater degree, by his progressive 
motion, which continually introduces him into new parts of the 
great Eetherial ocean. It would be, however, going too far to 
maintain that the brilliant orb is indebted to his motions for his 
power of emitting light, and that a state of absolute rest would 
reduce him to perpetual darkness. There can be little doubt 
that, by the property of diffusion which is common to all gases, 
the photosphere of the sun would gradually yield its place to 
fresh supplies of aetherial matter from space, according as its 
condition was changed by a manifestation of its luminous 
powers. 

Some writers, without professing to define the mode in which 
the light of the sun is generated, suppose that its develop- 
ment is exclusively due to his rotation around his axis. If 
this were the case, the planets Jupiter and Saturn would have 
more cause to be self-luminous orbs, for they have a far more 
rapid diurnal movement. In addition to this, the development 
of heat or light by mere mechanical action necessarily involves 
a loss of motion, and it is only when impeded by the resistance 
of a surrounding fluid that the mere rotation of a celestial 
body could be attended with any heating or illuminating power. 
But in this case, the chemical action which may be expected to 
take place in the ssthereal fluid, as it condensed around the 
great sphere, must wholly supersede the mechanical causes, 
and must, therefore, be considered as the main agency in ce- 
lestial illumination. 

The same objection may be urged with even more force 
against the theory of Professor W. Thompson, who regards 
the light of the sun as produced by the continual fall of me- 
teors to his surface from surrounding space. But to make the 
theory accord with observed facts, he finds it necessary to sup- 
pose that the meteoric masses, for the most part, revolve very 
close to the solar sphere, and that they are caused to fall to its 
surface in consequence of the impediments to their motions 



102 ORIGIN OF SOLAR AND METEORIC LIGHT. 

from the resistance of the interplanetary medium. A more 
just regard for observed facts, and for the principles of sound 
philosophy, might have led the ingenious author of this theory 
to conclude that the supposed meteors must have their orbits 
characterized by a singular uniformity of size and position, and 
that the same complicated and delicate arrangement must be 
extended to the innumerable suns of creation. But it is evi- 
dent that the resisting medium which is necessary to make 
such vast numbers of meteoric bodies reach the sun, would 
perform the work of illumination more effectively in their ab- 
sence, supposing it to possess properties which we should nat- 
urally expect to find in all fluids of a similar character. If, as 
I have shown reasons for believing, the brilliancy of suns is 
sustained by this universal medium, and is therefore dependent 
not on accidental but on determinate causes, science may be 
able to fathom the chemical, as well as the mechanical, phe- 
nomena of the heavens, and may trace the influence of wisely 
ordained laws not only in the preservation of worlds, but also 
in giving them the supplies of heat and light which they re- 
quire to become the seats of life and of intelligence. 



VARIABLE STAES AND THE SUN's SPOTS. 103 



THE VARIABLE STAKS AND THE SUN'S SPOTS. 



The abnormal character which natural phenomena occa- 
sionally present, or their apparent want of strict conformity to 
certain rules often furnishes the most effectual means for ob- 
taining an acquaintance with the causes from which they 
originate. Sir Isaac Newton was enabled to give a very con- 
vincing proof of his theory of gravitation, when he found that 
the elliptical motion of the planets is the inevitable result of 
solar attraction. But, to establish the truth of his discovery, 
it was found necessary to show that the slight deviation from 
elliptical motion, as exhibited by the several planetary bodies, 
was such as might be expected to arise from their mutual ac- 
tion in conformity with his celebrated law. It may seem ad- 
visable to apply a similar test to the present theory respecting 
the origin of solar light. The combustion of the space-per- 
vading medium on the surfaces of the greatest celestial 
spheres, is sufficient to account for the steady and perpetual 
emission of light from our own sun, and from the many thou- 
sand self-luminous bodies of our universe ; but it will be sat- 
isfactory to ascertain whether we can account, on the same 
principle, for the changes of brilliancy, which several of the 
stars experience, and which, in a few cases, is exhibited in a 
very decided manner. 

Of the several variable stars to which astronomical observa- 
tion has hitherto extended, Algol, in the constellation of Per- 
seus, has, or seems to have, the shortest period of variability. 
It passes through its several stages of brightness in a little less 
than sixty-nine hours ; and, for sixty-one hours of this period, 
it shines as a star of the second magnitude. During the re- 
maining eight hours, it declines to the fourth magnitude, and 
again resumes its former splendor. The star S Cephei un- 
dergoes a similar change in a period of one hundred and 



104 VARIABLE STARS 

twenty-nine hours, its brightness increasing* during nearly one 
third of the time, and taking about twice as long to diminish 
while it remains stationary only a few hours. 

But the variations of stellar light frequently assume a 
very complicated character. The star Mira, in the con- 
stellation of Cetus, is often so faint as to be scarcely visi- 
ble with the best telescopes, while at other times it is con- 
siderably brighter than stars of the second magnitude. The 
mean period of its variation has been estimated at three 
hundred and thirty-one days and twentj T hours ; but the 
changes frequently take place fifty days before or after the 
time at which they should be expected if they return at regu- 
lar intervals. In the maximum brightness which it reaches, 
it is sometimes little inferior to stars of the first magnitude, 
while, on other occasions, it scarcely exceeds those of the 
fourth, and there is even a still greater inequality in the de- 
grees of faintness to which it sinks at different times. Indeed, 
most of the stars having long periods of variability, are char- 
acterized by similar want of uniformity, not only in the 
recurrence of their changes, but also in the degree of magni- 
tude to which they sink and rise. Yet it appears, from the 
researches of Argelander, that, in this seeming disorder, law 
maintains its sway. The irregularities are always most decided 
after certain periods, and from the manner in which they recur 
after determinate cycles, we may reasonably hope that, like other 
celestial phenomena, they will be ultimately found to har- 
monize with the predictions of the astronomer. 

The intermission in the illuminating energy of these distant 
suns is not without analogies in our own system. The dark 
spots on the sun's surface must, obviously, have some influ- 
ence on the amount of light which he emits, and they are 
known to exhibit a periodicity in their appearance. The times 
at which the greatest numbers are visible, are separated by an 
interval of between eleven and twelve years, so that the sun 
may be regarded as a variable star, though the change of his 
light, from the presence of the spots, is too insignificant to be 
recognized in the worlds of other systems. 

It has been customary for astronomers to regard variable 



AND THE SUN'S SPOTS. 105 

stars as suns, which have a permanent darkness, or deficiency 
of illuminating power, on one side ; and the change in their 
appearance has been ascribed to a rotation, which alternately 
presents each side to our view. But, as the rotatory move- 
ment of the heavenly body is necessarily uniform, the changes 
in the apparent magnitude of these distant suns, if wholly 
dependent on this cause, would always recur at regular inter- 
vals, while the degrees of maximum and minimum brilliancy 
would be always exhibited on a uniform scale. Now such a 
state of regularity is not to be found in any of the variable 
stars, with the exception of Algol; yet, even in this case, the 
theory seems at fault, as it does not account for the fact that 
the deficiency of light is to be observed only in seven or eight 
hours out of sixty-nine. But, even if the hypothesis were 
adequate to explain the several phenomena connected with 
stellar variability, the facts and principles which modern 
science has revealed would call for its rejection. It appears, 
from Arago's observations and researches, that the light of 
suns and stars emanates from their luminous atmospheres, and 
from the diffusibility of different gases, there could be no 
material difference in the chemical composition of an atmo- 
sphere at different parts of the same sphere. From the princi- 
ples of stability, which have been explained in the first chapter 
of this work, it is evident that bodies so large as a sun could 
not have mountains of a sufficient height to interfere with their 
brilliancy. The result of observations on the sun's spots dis- 
countenances the idea that there can be a permanent cessation 
or diminution of the light-yielding action on any part of his 
surface. 

The intermission of stellar luster is ascribed by some to 
the eclipses, or the occultations of these remote suns by the 
planets which attend them. But the duration of such eclipses 
must always be much less than half the time of the revolution 
of the planet by which they are occasioned, and, if the change 
in the appearance of the stars proceeded from such an inter- 
ception of their light, their greatest brightness must continue 
unaltered for more than half the time. Yet such a peculiarity 
is not to be observed in any variable star, except in Algol, and 



106 VARIABLE STARS 

even in this instance the hypothesis seems wholly irreconcilable 
with observed facts. According to the observations of Arge- 
lander, as given in the third volume of Humboldt's Cosmos, 
this star, having increased in light for about an hour, remains 
uniformly bright for nearly the same period, and then begins 
once more, perceptibly, to increase. This phenomenon could 
not be accounted for by mere interception of light, which the 
transit of a planet might occasion. The periodicity of the 
solar spots, and of the changeable brilliancy of the stars might 
naturally lead us to infer that planetary action is not without 
its influence on the illumination of our own and other systems ; 
and, from the theory of celestial light, given in the preceding 
chapter, we may arrive at a knowledge of the effects which 
such bodies are capable of producing. A reduction in the 
pressure and density of the sun's luciferous atmosphere, and a 
consequent change in the development of his light might result 
from the attraction of his attendant worlds ; but this can lead 
to no decided consequences, except in cases where they move 
in very small orbits. But, as the great ocean of solar light is 
maintained by the universally diffused sether, the supply of fuel 
must depend, to some extent, on the progressive motion of the 
sun, and the position of the large planets. The attraction of 
the larger members of our system has frequently given 
comets a wide deviation from the paths they previously pur- 
sued, and it would be reasonable to expect that much of the 
aetherial fluid may be brought into a sun's way, or diverted 
from him by planetary influence. Such was the explanation 
which I suggested for the observed fluctuation of solar and 
stellar light, in my paper on the subject, which was read at the 
twenty-seventh meeting of the British Association for the 
Advancement of Science. But a more definite explanation of 
the manner in which planetary action operates, is given in an 
article which I contributed to the Philosophical Magazine for 
May, 1858. I shall introduce here the principal part of this 
communication, though much of it may not be very intelligible 
to the general reader. 

The action of gravity must cause the aether of space to in- 
crease its density, not only on the surfaces of suns and planets, 



AND THE SUN'S SPOTS. 107 

but also along the regions through which they have taken their 
rapid course. To prove this, it is only necessary to investigate 
the movements of an immense number of bodies not larger 
than common balls ; if they were scattered over an extensive 
tract of space, and were traveling in parallel paths, with the 
same velocity, until they came within the range of the sun's 
attraction. It will be readily seen that the several planes in 
which each of these bodies is urged by the solar power, 
must have a common intersection, in a line coinciding with the 
direction of their primitive motion. If the sun should travel 
through a region occupied by a similar assemblage of bodies, 
which had previously been at rest, he would compel each of 
them to describe hyperbolic orbits, all intersecting in the line 
of his progressive motion, and the space along this line would 
be most densely populated by the transient masses. Now the 
influence of elasticity would prevent the particles of the inter- 
planetary medium from describing similar orbits, but it would 
not change the planes of their motion, nor prevent all from 
intersecting the same line. The region from which the sun 
departs will accordingly be the focus to which the aether 
presses from all surrounding space, and here it must be con- 
centrated in the greatest quantity, having its density much 
augmented by the conflict of opposing currents of enormous 
extent, and moving with an immense velocity. 

Nor must it be supposed that the great elasticity of setherial 
particles would render the augmentation of gravity inconsid- 
erable. The pressure of an atmospheric column about forty 
miles high, makes the air we breathe several thousand times 
more dense than it would be if the height of the atmosphere 
were only one thousand feet above the level of the sea. 
Though the modulus of elasticity should be many million 
times as great in the luciferous ether as in our common air, 
we must recollect that it has to contend with the weight and 
inertia of a far more extensive mass of the rare fluid. The 
pressure of a column, whose height is commensurate with the 
range of solar attraction, must increase to a considerable ex- 
tent the density of the aether, while the repulsion of its parti- 
cles would require years to restore it to the regions from which 



103 VARIABLE STABS 

it was withdrawn. In like manner, large planets have the 
power of concentrating the eetherial medium in the regions over 
which they move, especially when their orbits are so wide, 
and the movement of the system so rapid, that their tracks in 
space deviate little from a straightjine. 

Had the planets moved in planes coincident with the line of 
solar motion, the columns of dense aether, which the larger 
ones leave behind them, would alternately pass over the sun, 
and cause a perceptible augmentation of his brilliancy. In 
many of the distant sj^stems, the track of the central body has 
so small an inclination to the planes in which its attendants re- 
volve, that each of these bodies takes some part in maintaining 
the great fire by which they are warmed and illuminated.. The 
stars x -Cygni and Mira exhibit, in their changes of magnitude, 
the peculiar phenomena which might be expected to result 
from such an arrangement. The mean time which the former 
star occupies in its course of variation, has been estimated at 
four hundred and six days ; but it sometimes differs several 
days from this amount, and in its maximum brightness it varies 
from the fourth to the seventh magnitude. It appears, how- 
ever, that these irregularities are most decided after a lapse of 
eight and a half and a hundred periods of variation ; and Ar- 
gelander shows that its changes may be calculated with toler- 
able accuracy by a formula similar to those used for the 
determination of planetary perturbations. The variable 
splendor of this distant sun must be ascribed to the influence 
of three planets ; one revolving around it in four hundred and 
six days, another in three thousand four hundred and fifty-one 
days, and the third in forty thousand and six hundred days ; 
and, it being confined to the planes which pass close to the line 
of the proper motion of the central luminary, they are enabled 
to affect the development of his light. The effects of planets in 
eondensing the etherial contents of space is also indicated by 
the fluctuating brilliancy of Mira ; for though there are great 
inequalities in the time and the degree of its variation, they 
are all periodical in their occurrence, and seem to be regulated 
by determinate laws. 

Of the other variable stars, the greater part are in like man- 



AND THE SUN'S SPOTS. 109 

ner characterized by such irregularities as might be expected 
to arise from the action of a plurality of worlds which attend 
them. In considering the conditions which planetary attrac- 
tion requires to produce such effects, it would seem that the 
members of other systems, like those of our own, have the 
planes of their orbits confined to a very limited range. There 
is, indeed, a great regularity in the decline and return of 
the brightness in some stars of short periods of variability ; 
but in these cases, the effect must be mainly dependent on the 
presence of a large planet which revolves in a small orbit, and 
alters, in a sensible degree, the attraction of the central sphere. 
Though this alteration might diminish weight and pressure 
only in certain localities; yet, from the conditions of equilib- 
rium, it must reduce the density of all parts of the sun's 
luciferous atmosphere, and lead to a corresponding diminution 
of his luster. Accordingly, when large planets describe orbits 
of small size and great eccentricity, they must exert a consid- 
erable influence on the illuminating power of the central orb, 
retarding the great conflagration when they are near, and 
allowing it to proceed with more vigor when they are most 
distant. 

In condensing, in the regions over which suns and worlds 
have passed, the aether must be withdrawn from more distant lo- 
calities, and there compelled to assume a more rarefied condition. 
Accordingly planets, whose planes deviate very much from the 
direction of the sun's progressive motion, only curtail the sup* 
plies of his setherial fuel, and diminish the amount of heat and 
light which he diffuses around him. It is only in this way 
that the members of our own system can influence the produc- 
tion of light in our central luminary, and the slight effects 
which they occasion are exhibited in the periodicity of the solar 
spots. In consequence of the great mass of Jupiter, his effect 
on the vast illumination must preponderate over those of other 
planets ; and it may be observed that the period of his revo- 
lution does not differ much from that at which the spots appear 
in the greatest numbers. 

Modern observations prove that the sun's spots are not only 
deficient in light, but also that they have a lower temperature 



110 VARIABLE STARS 

than other parts of his surface. If, therefore, they are caused 
by exhalations from the solar mass, we may reasonably conclude 
that the gas which rises from his internal regions, only serves 
to interrupt his heating and illuminating action. If they are 
to be regarded as indications of a dark atmosphere beneath 
the luminous one, the mere situation of this dark envelope 
would be an evidence that it was derived from the sun him- 
self, and would invalidate the opinion that he emits combustible 
gases. Indeed, if his heat and light were maintained by gases 
expelled from him by the high external temperature, the fuel 
must be supplied in the greatest abundance, when the fire is 
most violent, and the great conflagration should steadily in- 
crease until the solar mass was entirely consumed, or it should 
continually decline until he sunk to perpetual darkness. But 
the greatest chemical operations in nature are not left to the 
direction of the most capricious agencies. Supported by the 
medium which pervades all space, and burns in accordance 
with fixed laws, the brightness of suns can only fluctuate in 
obedience to the influence of attendant worlds, and future ob- 
servations on the variable stars, with better photometric instru- 
ments than we can now command, may yet enable astronomers 
to trace the movements of planets in systems too distant to be 
explored with the telescope. 

In the preceding communication, I did not undertake to 
define the precise manner in which the features of the sun 
change with the increase or decline in the supplies of his 
setherial fuel. But my late researches seem to furnish a very 
plausible explanation of the several observed facts, without the 
introduction of any new hypothesis. From the phenomena 
which the dark spots exhibit, we have just grounds for regard- 
ing them as occasioned by violent atmospheric movements, 
similar to tornados on our own planet ; while it appears, as a 
legitimate deduction from theory, that the solar storms would 
be more rare than usual when the sun is advancing into the 
denser realms of sether, and that they must increase in fre- 
quency when he is departing from those localities and entering 
a more rarified eetherial fluid. In the first case, his permanent 
atmosphere, which takes no part in the development of his 



AND THE BUN'S SPOTS. Ill 

light, must be heated most extensively in its uppermost strata, 
and the expansion which it undergoes at this point, instead of 
disturbing its equilibrium, will only bring it into a state of 
greater repose. But when the supply of setherial fuel from 
space becomes less, and the fierce heat declines at the surface 
of the ocean of incombustible gases, the high temperature pre- 
viously imparted to its lower parts must cause the most violent 
commotions. Of the opposite effects which heat produces, 
according as it is applied at the bottom or top of a liquid or 
gaseous mixture, any one may convince himself by easy 
experiments; and the same laws of equilibrium must evidently 
exist in the sun, and his tornados, or aerial disturbances, must be 
more frequent when the heat of his photosphere loses its pre- 
ponderance over that of his surface. 

The occurrence of storms, or swellings in the sun's atmos- 
pheric envelope, will account satisfactorily for the peculiarities 
exhibited by the spots. From the perfect transparency of his 
dark atmosphere, combined with the effects of flame in inter- 
cepting the range of vision, the spots appear as hollows in the 
sun's disk. Gas, issuing from the interior of the great globe, 
might, indeed, exhibit the same appearance, but the depend- 
ence of the spots on atmospheric movements is shown by the 
periodicity which they exhibit, and by the change which the 
observations of Peters have revealed in their heliocentric posi- 
tion. 



112 TEMPORARY STARS, 



TEMPORARY STARS. 



The production of light on the celestial bodies becomes a 
more interesting subject for contemplation, in consequence of 
the paroxysmal character which it occasionally assumes. The 
sudden appearance of bright stars in the firmament, and their 
subsequent decline and extinction, are circumstances which 
have long served to call forth the surprise of astronomers. So 
very rarely are such phenomena to be witnessed in the skies, 
that they can only be regarded as indications of occurrences 
which are very rare, even in the numerous host of bodies by 
which space is populated. According to Humboldt, the 
number of temporary stars observed during the last two 
thousand years does not exceed twenty-two, or about one for 
every century. Of those visible in ancient times, the most 
celebrated was that which appeared in the age of Hipparchus, 
and, according to Plin}', prompted that astronomer to form his 
catalogue of the fixed stars. 

The year 1572 was rendered remarkable in the annals of 
astronomy by the appearance of one of these new stars in the 
constellation of Cassiopeia. It was seen by Tycho Brahe, on 
the 11 th of November, which was probably the first night on 
which it was visible, and even then it was far superior in 
brightness to stars of the first magnitude, so that it attracted 
the interest and curiosity even of persons wholly unacquainted 
with astronomy. So great was the intensity of its light that it 
was visible during the daytime, and at night could be seen 
through clouds of inconsiderable density. On the next month 
after its first appearance, a sensible diminution was observed in 
its light, and, by a constant loss of brilliancy, it sunk, in four 
months, to the rank of stars of the first magnitude. It con- 
tinued to undergo a constant and regular decrease of bright* 



TEMPORARY STARS. 113 

ness, until it ceased to be visible in March, 1574, after havino- 

o 

shone seventeen months. 

Another star, similar to that described, and almost equal to 
it in splendor, appeared in the constellation of Ophiuchus, on 
the 10th of October, 1604, and continued visible until the 
month of February, 1606. It also underwent a regular de- 
crease in its apparent magnitude, being most brilliant on its 
appearance, or, at least, a few days afterward. Indeed, Kepler, 
who had the rare opportunity of witnessing two of these stellar 
curiosities, regards it as a general rule that they are invariably 
most brilliant on their first appearance, and the validity of this 
conclusion has been recognized by Humboldt, who attaches to 
it considerable importance. "The circumstance," says this 
writer, "that almost all these new stars burst forth at once with 
extreme brilliancy, as stars of the first magnitude, and even 
with still stronger scintillations, and that they do not appear, 
at least to the naked eye, to increase gradually in brightness, 
is, in my opinion, a singular peculiarity, and one well deserv- 
ing of consideration." 

But, though temporary stars attain their greatest brightness 
so very quickly, an^ then slowly decline, they do not invaria- 
bly manifest the conspicuous appearances which were witnessed 
in those of 1572 and 1604. In the year 1600 there appeared 
in the constellation of Cygnus a new star of the third magni- 
tude, which, after some fluctuations in its light, sunk to the 
sixth magnitude, and as such still remains in the firmament. 
Another of the third magnitude was observed in the constella- 
tion of Vulpes in 1670, and, after some changes in its bright- 
ness, slowly disappeared in 1672. The last instance of these 
phenomena was witnessed in 1848, when Mr. Hind discovered 
a star of the fifth magnitude, which continued to decline until 
it finally faded away from the powers of telescopic vision, and 
may be considered extinct. In the several cases in which these 
objects have been submitted to careful observation, no change 
has been detected in their positions — a fact which proves that 
they must have been as distant as many of the fixed stars, and 
the temporary conflagration which rendered them visible from 
such unfathomable depths of space, must have rivaled, or even 



114 TEMPORARY STARS. 

surpassed, that which maintains the perpetual heat and light of 
our sun. 

These extraordinary appearances have called forth much 
speculation respecting the nature of the great physical events 
in the vast realms of creation. To many they seem indications 
of catastrophes involving the destruction of systems of worlds 
which are favored with a less degree of security than is to be 
found in our own departments of space. JSewton regarded 
the ephemeral displays of light as caused by the fall of a 
comet to some distant sun ; and he contended that the fall of 
^he comet of 1680 to our own luminary would give it a heat- 
ing power sufficient to depopulate our planet. But as nearly 
all the temporary stars finally disappeared, there seems to be 
no adequate ground for regarding them as permanent suns ; 
and if cometary matter were inflammable, it is difficult to con- 
ceive how the comets of 1680 and 1843, while sweeping so 
close to the sun, could have escaped being set on fire, and pro- 
ducing the desolation which Newton expected in future times. 
But it is due to the profound philosopher to state that this opin- 
ion was only given as a conjecture, in a conversation with one 
of his friends, and probably it was as reasonable as could be 
expected at an age when chemistry was in so imperfect a con- 
dition. My present object is^ to trace the catastrophes which 
the operation of known causes would render inevitable in the 
age of worlds, and which must be occasionally witnessed 
among the great multitude of systems located in boundless 
space. 

Had the celestial bodies moved through an absolute vacuum, 
and been exempted from the effects of tidal action, no perman- 
ent change could occur in their mean motions, or in the size of 
their orbits. But the universally diffused medium constantly 
impedes the movement of the planets, and causes the dimen- 
sions of their orbits to undergo a corresponding reduction. As 
it may be possible that the effects of this resistance on the pri- 
mary planets might be, in a great measure, counteracted by the 
waste which is suspected in the mass of the sun, I shall not at 
present disturb the pleasure which many persons feel in believ- 
ing that our earth and many other large worlds are to be fa- 



TEMPORARY STARS. 115 

vored with the boon of an eternal existence. But in the sec- 
ondary systems, at least, there can be no such compensating 
cause, and the ellipses in which the second-rate worlds move, 
slowly reduce in dimensions to an extent which could only be per- 
ceptible after several millions of centuries. The effects of these 
slow changes must be to make each satellite reach the primary ; 
and after many millions of years have passed away, the first 
attendant of Jupiter will be revolving very close to his surface, 
and passing through the last stage of its existence. Were it 
capable of preserving the planetary form until it arrived at Ju- 
piter's surface, it would sweep through his luciferous atmos- 
phere, and a grand meteoric exhibition would be inevitable. 
Experience furnishes no ground for supposing that a sun should 
have its brilliancy excessively augmented on the reception of a 
comet ; but we have a certain basis for tracing the conse- 
quences of the great events which terminate the career of pri- 
mary or secondary worlds. From the light which meteors 
emit in falling to the earth, and the extraordinary brilliancy of 
those which move in a horizontal direction, we may form some 
idea of the grand meteoric illumination occasioned by a globe 
two or three thousand feet in diameter, if it was making its 
last revolution around Jupiter, within a few thousand miles of 
his surface. As the eetherial atmosphere of this great planet 
must be far more dense and extensive than that surrounding 
the earth, it must give more scope for meteoric brightness, and 
the mighty meteor which one of its satellites may be expected 
to form, at some distant period, would accordingly send the 
most extraordinary floods of light into space. 

In a vast number of created objects, we may reasonably hope 
to find occasional instances of the several physical events, which 
any one might experience during a course of innumerable 
years. If a single family of worlds could afford one instance 
of these sublime catastrophes in the course of 4,000,000,000 of 
years, we may naturally conclude that, on an average, one 
might be exhibited every century in our universe, supposing it 
to contain 40,000,000 of planetary systems. But it is not among 
the secondary planets alone that we are to look for instances 
of these extraordinary occurrences. That our universe con- 



116 TEMPOEAEY STAES. 

tains a great number of dark central spheres, with worlds 
revolving around them, has been maintained by several dis- 
tinguished astronomers ; and, according to Laplace, these dark 
bodies are probably as numerous as the visible stars. The ex- 
istence of these non-luminous tenants of space, has been also 
maintained, on various grounds, by Msedler and Bessel, and the 
same doctrine derives much support from all the knowledge 
we can obtain of the condition of the known planetary sys- 
tems, and the laws which govern the productions of solar 
light. We have sufficient ground to conclude that no sphere 
could be permanently self-luminous unless it contained at least 
over fifty thousand times the quantity of matter in the earth, 
while a much smaller size and weight would entitle it to the 
government of a retinue of worlds. Throughout the vast do- 
main of space there must be a great diversity in the scale of 
celestial architecture, and many central bodies must be attended 
with planets which they are incapable of supplying with per- 
petual floods of heat and light. 

The dark systems which compose our universe, must remain 
long hidden from our view, and no indications could be gained 
of their existence until one of their worlds, after reducing 
its orbit for innumerable ages, came so near the central body 
as to cause the development of meteoric light to take place on 
a gigantic scale. Such an occurrence would proceed in a 
manner somewhat different from what might be first supposed. 
From the principles which I have explained in the chapter on 
planetary instability, it is evident that a satellite, consigned to 
a very small orbit, will necessarily undergo a sudden dismem- 
berment, unless it were about seven times as dense as its pri- 
mary ; for, in this case, it might remain unbroken until reach- 
ing the surface of the latter body. The member of a dark 
system will likewise undergo a sudden dilapidation, when a 
resisting medium gradually introduces it into an orbit too 
small for its stability. Accordingly, instead of terminating its 
career as one vast meteor, the doomed world would fall into 
an innumerable host of meteoric masses, which would evolve 
far greater floods of light by sweeping through a more ex- 
tensive portion of the aetherial atmosphere encompassing the 
central body. _.,..,. v 



TEMPORARY STARS. 117 

It is easy, by the principles of physical science, to deter- 
mine every circumstance attending such great catastrophes, 
and a comparison with the results of observation will be suffi- 
cient to show that the temporary stars may be, with propriety, 
regarded as one of the vast meteoric scenes which must 
necessarily accompany the fall of a world from its planetary 
condition. In this way alone we can account for the rapidity 
with which these ephemeral suns start forth into their greatest 
brilliancy. It might be shown that a satellite as dense as the 
earth, on entering an orbit in which it would be reduced to an 
unstable condition, would complete its revolutions in about five 
hours. Supposing it revolved around a central sphere equally 
dense, and one hundred thousand miles in diameter, it would 
move at the rate of seventy miles a second, while a large por- 
tion of the resulting fragments had a far greater velocity. As 
soon as the planetary structure fell in ruins, and the meteoric 
aetion was commenced by the fragments sweeping nearest to 
the central sphere ; the great heat and pressure would cause 
their rapid subdivision into smaller masses, and the evolution 
of light would attain its greatest magnificence. No other ex- 
planation can be given for the extraordinary brilliancy of 
temporary stars at an early stage of their visibility. 

The subsequent decline and disappearance of these ephem- 
eral suns corresponds to the results which the present theory 
would enable us to predict. In treating on the instability of 
Satellites, in a former chapter, I have shown that the fragments 
of such a wreck would ultimately have their orbits changed to 
circles, and arrange themselves into the form of a ring similar 
to that around the planet Saturn. As this change advances, 
the medium will be made to partake of their motion, and be- 
come insensible to their pressure, so that the evolution of light 
will undergo a corresponding diminution, until it finally ceases 
forever. I may remark here, that the rotation which the 
fragmentary host give to the setherial atmosphere of the larger 
sphere, would have a tendency to bring their own orbits into 
a circular form. During its continuance, the meteoric action 
would be most decided at the principal perihelion point of the 
fragmentary orbits, and as the position of this will occasionally 



118 TEMPORARY STARS. 

change by apsidal motion, the great body of light will be oc- 
casionally screened from us by the interposition of the central 
body. This will account for the fluctuation of brightness, and 
even the temporary extinction which some of the temporary 
stars experienced before they disappeared for ever. 

By tracing the necessary results of known causes, we thus 
arrive at a satisfactory explanation of one of the most myste- 
rious phenomena of the heavens. From the difficulty of con- 
ceiving how so great a conflagration could at once spring into 
existence, different views have been taken in regard to the 
temporary stars. Among other extravagant hypotheses in 
relation to them, it has been suggested that they are stellar 
bodies, which sometimes approach so near our system that they 
manifest a most extraordinary brilliancy, and then retire so far 
that they gradually become invisible. A velocity about twenty 
thousand times as great as that of light would be requisite to 
enable a body to take these rapid flights, and a sphere as dense 
as the sun, and having a diameter four million times as great 
would be scarcely capable of controlling so violent a movement. 
A body of so great a mass could not exist in our universe 
without displaying its attractive power, in imparting an equally 
rapid movement to all the siderial host. But I may obviate 
the necessity of further objections, by stating that all observa- 
tions on temporary stars have hitherto failed to detect any 
change in their apparent position. 



ELECTOICAL LIGHT AND THE AURORA BORELFAS. 119 



ELECTRICAL LIGHT AND THE AURORA B0REALI3. 



It appears, from the foregoing pages, that the production 
of light in the heavens is governed by the same uniform laws, 
whether it manifests a steady, a variable, or even a paroxysmal 
character. The whole stellar host, including even the tempo- 
rary and variable stars, have their luciferous action maintained 
by the all-pervading eether, which is ever equally ready to 
burn around the insignificant meteor, and around the mighty 
sun which presides over many worlds. The mere appearance 
of a shooting-star must, therefore, be regarded with peculiar 
interest, as it gives intelligence of the vast magazine of lucif- 
erous aether, designed for the illumination of the worlds of our 
universe. Perhaps other evidence may be obtained that the 
subtile fluid, which acts so extensive a part in celestial econ- 
omy, exists on our planet, and takes part in several mysterious 
operations connected with animal and vegetable life. But we 
must recollect that it is only within the last century that chem- 
istry has made us acquainted with the presence of the gases 
which are now recognized as constituents of the atmosphere ; t 
and, for a long time, this science must fail to afford any means 
of detecting those finer elements which cannot be confined by 
any vessels now used in chemical experiments. 

There are, however, some electrical phenomena which seem 
to indicate that there exists around us a luciferous fluid entirely 
distinct from the known constituents of the air. When elec- 
tricity is transmitted through the vacuum of an exhausted 
receiver, it produces a luminous appearance, and the effect 
becomes more decided in proportion as the air is removed. It 
seems extremely difficult to imagine that the light could be 
produced without some matter, or that its development re- 
quired nothing besides the mere absence of the air. The 
phenomenon seems evidently to depend on the presence of a 



120 ELECTRICAL LIGHT 

subtile, light-producing fluid, which is rendered more sensitive 
to electrical influence, when the more gross aerial elements are 
removed from the scene of action. The electricity may pro- 
bably induce, in a direct manner, the chemical changes from 
which the light originates, or may cause the few aerial particles 
in the receiver to vibrate with more velocity through the par- 
tial vacuum, and give the aether the pressure necessary for 
the luciferous operation. 

The nature and the cause of this luminous phenomenon can 
be better understood from a similar experiment which nature 
performs, on a far more sublime scale, in the polar regions. 
According to the prevailing opinion at the present day, the 
Aurora Borealis is occasioned by the passage of electricity 
through the rarefied atmosphere, above the colder parts of the 
earth. But this theory, though maintained by several distin- 
guished scientific men, has been controverted by others ; and 
Professor Olmsted has recently adduced several facts, to show 
that the Aurora is of cosmical origin, and that it is caused by a 
luminous fluid which continually flows into the polar regions 
from celestial space. Now this influx of cosmical matter could 
not be maintained except the fluid was permitted to depart 
from the earth at some other locality, and it is difficult to see 
how matter projected from the sun or some other department 
of the heavens, should always fall on the same part of our 
planet. But the facts which have been adduced in support of 
the two rival theories, may be all satisfactorily explained by 
supposing that the earth's rotation causes the luciferous fluid 
of space to flow in around the poles, while it escapes at the 
equatorial regions, and that it is rendered luminous by the 
currents of electricity which travel to the coldest parts of the 
earth, along the verge of our atmosphere. 

By considering the phenomena in this point of view, we are 
enabled to account for a fact which none of the rival theories 
enable us to explain. It has been observed that, during an 
Aurora, certain parts of the vault of heaven, which were not 
previously illuminated, light up, and continue luminous when- 
ever a shooting-star passed over them, (See Cosmos, vol. 1, page 
127, Harper's ed.) From this it would appear that the same 



AND THE AURORA BOREALIS. 121 

Iuciferous aether is concerned in auroral and meteoric light, and 
that the pressure of a rapidly moving meteor stimulates it to a 
chemical action, which is subsequently kept up for a short time 
by the influence of the electricity. 

It has been maintained by some astronomers that solar light 
is of a similar electrical origin, and that it may be regarded as 
an Auroral phenomenon, exhibited on a colossal scale. But it 
must be recollected that a perpetual manifestation of electricity 
requires the same conditions as the perpetual development of 
light, and that constant electric currents on the sun could be 
only kept up by some incessant mechanical or chemical action. 
If they were of voltaic origin, the materials of the sun would 
be soon expended in giving them existence. If frictional 
electricity were the main cause of the great illumination, it 
must be necessarily attended with a loss of motion in the solar 
mass. In an article published in the Philosophical Magazine, 
for May, 1858, Professor Draper has inferred, from the result 
of optical experiments, that the sun's light is to be regarded 
as the offspring of direct chemical action, and that electricity 
does not appear to have any part in its production. In the 
same article he has shown that a diversity of colors may be 
exhibited by light, from the combustion of the same materials, 
according as the action is weak or vigorous. From this we 
may infer that there is no uniformity of color required in the 
different stars, even though the brilliancy of all is maintained 
by the same aetherial fluid, and that the color of these distant 
suns must depend on the composition and the extent of their 
dark or permanent atmospheres. 



122 CONCLUSION. 



CONCLUSION. 



To prove the stability of the solar system has been the lead- 
ing object to which the labors of physical astronomers have 
been hitherto directed, and it appears, from their researches, 
that the several known planets must continue their present 
career of existence during innumerable ages. Such results 
are indeed calculated to impress the mind with a due appreci- 
ation of the grand designs displayed in the architecture of the 
heavens, and if those mighty domicils of life and intelligence 
could have secured to them the blessings of heat and light for 
an equally long period, the plan of creation may be pro- 
nounced complete. But, from the crude theories hitherto pro- 
posed in regard to the constitution of the sun, it would seem 
that this great body must, in a few centuries, sink to eternal 
darkness ; that his worlds will then be doomed to pass through 
a night of endless duration, and that the wise arrangement 
which secures to them an immeasurable age, will only serve to 
consign them to a state of perpetual inutility. 

It appears, however, that the admirably effective means for 
maintaining the harmony of creation, is afforded not only by 
the mechanical, but also by the chemical operation of the heav- 
ens. By the inexhaustible supply of gether, which fills illimita- 
ble space and burns on the largest spheres, the light of suns, 
'if not of eternal duration, must continue to supply the demands 
of his worlds for an immense number of ages. The suns of 
cur universe are utterly insignificant in comparison to the vast 
space from which they continually draw their supplies of lu- 
ciferous matter, and the several distant universes, or stellar 
congregations, are even small compared with the starless space 
by whieh they are separated. If, as there is reason to suspect, 
the aether of space is restored to its luciferous condition, by the 
action of light or some other cause, the duration of solar 
brilliancy must be inconceivably long, if not absolutely eternal. 



CONCLUSION. 123 

In the foregoing pages I have abstained from entering into 
an examination of the merit of the rival theories respecting 
the nature of light, or the manner in which it is propagated. 
The undulatory theory has been confirmed by so many expe- 
riments, during late years, and has met with such favor among 
scientific men, that I shall not hesitate to adopt it. Ac- 
cording to this theory, light is propagated by the undulations 
of a rare fluid, in the same manner as sound is conveyed by the 
air. But, as light travels about one million times as rapidly 
as sound, the medium which conveys it must be 1,000,000,- 
000,000 times as elastic as a portion of air having the same 
amount of matter. As a medium having so high a degree of 
elasticity can have but little more density on suns and planets, 
than in the inter-stellar regions, it cannot be identical with the 
tether which sustains the perpetual brilliancy of suns, and ac- 
cordingly the fluid which is concerned in producing solar light 
is wholly different from that which conveys it with such an 
inconceivable velocity. Could chemistry lend the means of 
examining the more subtile forms of matter, we may find a far 
greater diversity in the elementary constitution of the ajtherial 
contents of space, than is now known to exist in the solid and 
liquid parts of our own planet. 

The effects of spongy platinum in inducing chemical action 
in a mixture of oxygen and hydrogen may serve to convey a 
good illustration of the influence of large spheres in kindling 
the aether of space into a state of chemical activity. But in 
the latter case, the result cannot be manifested without the 
enormous attractive power which a large sphere can exert ; and 
the fact that all the suns are vast accumulations of matter must 
be regarded as opening the most fertile source of information 
respecting the stellar universe. From the laws which govern 
the production of celestial light, we may look upon the self-lu- 
minous condition of the celestial bodies as evidence of their 
vast magnitude; and the millions of stars which the telescope 
brings to our view will convey to us a more sublime idea of the 
magnificence of creation. It is true, indeed, that the conclu- 
sion to which our theory leads does not differ materially from 
that now held by astronomers, most of whom are willing to ad" 



124 CONCLUSION. 

mit that the fixed stars hold the rank of suns. But this doc- 
trine, though generally embraced, has been commended to ac- 
ceptance more by the intellectual pleasure it affords the mind 
than by the strength of the proof which has been adduced to 
support it. The want of sufficient evidence in this department 
of astronomy was recently felt in the controversy respecting the 
nature of the stars, when their great size was questioned by 
the anonymous author of the " Plurality of Worlds." 

The researches in the preceding pages enable us to proceed 
to the study of celestial phenomena with more confidence, as 
we may regard the luminous condition of stellar bodies as an 
evident indication of their greatness. There is not the slight- 
est reason to believe that the self-luminous character displayed 
by the fixed stars could ever be associated with small masses. 
The shooting-stars do not manifest their lucid appearance until 
they approach the earth, and evolve light from our luciferous 
atmosphere, Perhaps the tails of comets may be an instance 
in which light is produced without the intervention of a large 
amount of matter; but such a light could not be visible from 
the region of the stars. Great meteoric displays may occasion- 
ally occur in space, and a small sphere may, for a time, present 
the appearance of a sun; but in the natural course of celes- 
tial events these occurrences must be rare and of short contin- 
uance : so that we can, without any serious error, regard all 
stellar bodies as holding the same rank as our sun. 

By a knowledge of the principles on which the great luci- 
ferous processes of nature are conducted, we are enabled to 
form more just views respecting the so-called nebula which 
have given rise to so much controversy among astronomers. 
In different parts of the sky the telescope shows faintly-shining 
masses having some resemblance to luminous clouds ; but as 
no change has been detected in their apparent positions, they 
must be at least as distant as the nearest sidereal bodies. It 
was the impresssion of many astronomers that these objects 
were composed of nebulous matter, or extremely attenuated 
cosmical vapor, which, by continually parting with its heat, is 
slowly condensing, and is ultimately destined to form suns 
and planets. But on being examined by more powerful teles- 



CONCLUSION. 195 

copes, several of these nebulae have been found to consist of 
innumerable stars, and the impression that all are of a similar 
character has become general among astronomers. 

Nor must it be supposed that the stellar objects into which 
nebulae have been resolved are inferior in rank to our own sun, 
or to the bright stars which impress so decided a character on the 
scenery of our skies. Indeed, the anonymous writer I have 
already alluded to, in his attempts to sink the glories of the 
heavens to insignificance, contends that the bright points into 
which the nebulae have occasionally resolved are mere dots or 
lumps of light. But as I have shown that the perpetual emis- 
sion of light from such small objects is physically impossible, 
the idea of luminous dots, star-dust, and even self-luminous 
vapor, must be forever abandoned. There seem to be no ade- 
quate grounds for regarding the zodiacal light as evolved from 
self-luminous matter ; and in a paper read before the meeting 
of the British Association in 1854, I have shown reasons for 
believing that it is reflected from the extensive zone of our sys- 
tem, which the smallest meteors select for their orbits around 
the sun. (See Reports of the Proceedings of the Meeting for 
1854, p. 26.) 

The stellar constitution of the nebulae is also indicated by the 
facts which the telescope has made known respecting the con- 
stitution of the sidereal universe. The vast starry host, of 
which our sun is a member, occupies a tract of space, the 
length and breadth of which are estimated as seven hundred 
times the average distance of the nearest fixed stars, while the 
depth is only about one-fifth as extensive. This vast expanse 
is tenanted by between thirty and forty millions of suns, and is 
surrounded by a space either wholly starless, or containing a very 
scanty population of stellar bodies. There is little doubt that 
this vast collection of shining orbs would appear as a nebula if 
viewed from so vast a distance that the individual stars would 
be incapable of producing a distinct impression on the eye. 
The difficulty of resolving a nebula must, in some measure, 
depend on the number of stellar objects of which it is com- 
posed. As the light which several millions of stars emit must 
render the eye insensible to the impressions of a single one, 



126 CONCLUSION. 

we may reasonably infer that those nebulae in which they are 
almost innumerable must be, in many cases, irresolvable by 
any powers of the telescope. To a cause of this nature we 
may ascribe the fact that the great nebula of Andromeda has 
been found by Mr. Bond to consist of small stars on its bor- 
ders, while all attempts to prove by observation the stellar con- 
stitution of the central part have been hitherto fruitless. 

From the progressive movement of our system, some astron- 
omers have attempted to estimate the force of gravity to which 
jt is subject from the combined influence of surrounding suns 
and worlds. This would give us some idea of the amount of 
matter contained in the several systems of our universe ; but, 
unfortunately, the data which observation has hitherto fur- 
nished is far from being sufficient to conduct us to any satis- 
factory results. In addition to this, the physical theories can 
only apply to cases far more simple than those presented in na- 
ture. It might be easily shown that, if 100,000,000 spheres 
each containing as much matter as our sun were distributed 
uniformly over a spherical space 9,500,000,000,000,000 miles 
in extent, each of them would occupy a period of 100,000,000 
of years, either in revolving around their common center of 
gravity, or in making a double vibration through that point. 
As such a vast tract of space is much less than that assigned 
to our universe, or even to many of the clusters of stars, the 
author of the Plurality of Worlds is certainly mistaken in re- 
^ardino- the slow movements of these bodies as an indication 
of the weakness of the attractive power which holds them to- 
gether. 

The error of the writer in the present case must be ascribed 
to his incorrect deductions from the theory of central forces. 
" If," he says, "our sun were thus broken into fragments, so 
as to fill the sphere girded by the earth's orbit, all the frag- 
ments would revolve around the center in a year. JSTow, there 
is no symptom in any cluster of its parts moving as fast as this ; 
and therefore we have, it would seem, evidence that the groups 
(of stars) are much less dense than would be the space so 
filled with the fragments of the sun." The statement contained 
in the first sentence is correct, or at least would be so, if there 



CONCLUSION. 127 

were an equable distribution of the solar fragments throughout 
the spherical space bounded by the earth's orbit. But the in- 
ference in the remainder of the quotation cannot follow from 
it, except it be first shown that no cluster of stars occupies a 
greater space than that assigned to the earth for its periodical 
revolutions. 

Besides revealing the great magnitude of the fixed stars, 
science is called to perform a more difficult task. It must be 
interesting to know whether those brilliant orbs which rival 
our sun in greatness are destined to fulfil equal]}- useful ends, 
byhavin<>- their light appropriated to the wants of surrounding 
worlds. Though the existence of planets around distant suns 
has been maintained on moral grounds, the information which 
science is capable of affording on the subject cannot be con- 
sidered as wholly unnecessary. Notwithstanding the revela- 
tions which the telescope has made in our own system, we can 
never expect that it will ever bring to our view the planetary 
bodies attending other suns, except in the rare cases in which 
they are self-luminous. But this instrument is far from being 
the only, or even the most effective, means which science has 
furnished for discovery. The genius of Leverrier was enabled 
to determine the existence of a remote member of our system 
from the slight effect it manifested in disturbing the movements 
of a neighboring world. 

But the influence of hidden planets is frequently attended 
with an effect far more conspicuous than that which gave Le- 
verrier the clue to his wonderful discovery. In another place, 
I have shown that the larger attendants of the sun disturb, in 
a sensible degree, his luciferous operation ; that the revolution 
of Jupiter is nearly, if not exactly, equal to the period at which 
the solar spots appear on the most extensive scale ; and that the 
changes in the changeable brilliancy of the fixed stars may be 
traced to the influence of surrounding worlds. In the case of 
X Cygni, the existence of three large planets is very decidedly 
revealed by the fluctuations of brilliancy which it exhibits, and 
perhaps many other attendants will be made known in this and 
other remote systems, when the progress of science and art 
brings better photometric methods within the reach of the as- 
tonomrer. 



128 CONCLUSION. 

The degree of variation which the light of stellar bodies un- 
dergoes will enable us to form an opinion respecting the size of 
the planets which attend them ; and we can arrive at a more 
definite idea as to the size of their orbits. Had all suns ex- 
actly the same quantity of matter, we may easily tell, by means 
of Kepler's third law, the distance of his several planets, if 
the periods of revolution were known. But suns are not 
strictly equal in size, and as we can only determine the lowest 
mass which they must possess to be self-luminous, we must 
content ourselves with calculating the lowest dimensions which 
may be assigned to their orbits. Although only a small part 
of the stellar host can furnish indications of their attendant 
worlds, yet, when we consider that it is only in very rare cir- 
cumstances that their light can fluctuate by the influence of 
planetary attraction, we may safely assert that m#st of the dis- 
tant systems have as numerous a population of large worlds as 
can be found in our own. 

In respect to size or weight, no immutable relation has been 
fixed between the several planetary bodies and the central 
spheres which direct their movements. The sun is about 
25,000,000 time as large as Mercury, and more than a thou- 
sand times as large as Jupiter. The latter planet has over ten 
thousand times as much matter as his largest satellite, while 
the earth has only eighty times as much as the moon. In 
many parts of the universe, however, the great disproportion 
between the central body and its attendants disappears, and 
this seems to be the case with the double stars. In these dis- 
tant systems the development of planets has taken place on such 
a large scale, that many of them have a size sufficient for the 
rank of a permanent sun. To calculate the influence of plan- 
etary disturbance in cases where the attractive power of the 
central mass, has no great superiority over that of its attend- 
ants, is more than physical astronomy could effect in its present 
condition. There can be, however, little doubt that the 
enormous disturbing forces in such cases would be attended 
with far greater deviations from circular motion than we ob- 
serve in the revolution of the primary or secondary worlds of 
our own system ; and I may here remark that so far as obser- 



CONCLUSION. 129 

vation has yet discovered, the orbits in which the double stars 
move, are almost as eccentric as those which comets describe 
around our sun. 

But, if in some systems, many planets rise to the rank of 
suns, we naturally expect that, in others, the scale of celestial 
architecture would be so much reduced that many central 
bodies might fall short of the vast size which may be required 
for their perpetual brilliancy. That numerous planets in space 
revolve around dark spheres incapable of supplying- them with 
light can scarcely be doubted. To suppose that all the celes- 
tial bodies, which control the movements of planets, have the 
size and mass of suns, would be as unreasonable as to expect 
that distant worlds have all their rivers sufficiently large to be 
navigable, and all their islands sufficiently extensive to serve 
as the seats of mighty empires. We are, therefore, con- 
strained to believe, with Laplace, that the dark occupants of 
space are in all probability as numerous as the bright suns, 
though we may differ from him in the belief that they have an 
equal magnitude. 

Perhaps this doctrine may be opposed on moral grounds, by 
many persons who justly regard all created objects as destined 
to serve some useful end, and who suppose that no great pur- 
pose' can be fulfilled by worlds which pass their existence in a 
night of eternal duration. But, though a certain degree of 
usefulness may be regarded as the necessary attribute of all 
the great works of creation, we cannot admit that they are 
eternally useful, or that they have their utility manifested at 
all times. During the early geological ages, the earth ex- 
hibited little adaptation for the great purposes which it was 
destined to fulfill in a future period ; and, even at the present 
day, the moon, from the want of air and water to fit it as a 
habitable globe, has its utility limited to the more humble 
office of furnishing our globe with a scanty supply of light. 
All material forms have their stages of preparation to pass 
through before they become instrumental in promoting the 
great works of nature. In the never-ending changes which 
transpire in creation, the relation between the celestial orbs 
must alter with the march of time, and the obscure tenants of 
9 



130 CONCLUSION. 

space, which are now incapable of heating or illuminating their 
worlds, may yet shine forth as the stars or suns of a future 
universe. 

" r , In another chapter I have shown the important information 
which we obtain respecting these obscure bodies from the ap- 
pearance of temporary stars. These celestial curiosities were 
regarded by Laplace as- the chief evidence of dark central 
spheres, but from the manner in which the great flood of light 
is so rapidly developed, it is evident that these bodies are not 
solitary occupants of space. The great illumination necessa- 
rily involves the fall of one of their planets from the stage of 
existence, and when we reflect that, in a single system of 
worlds, such an event could be scarcely expected in the course 
of many millions of years, we are compelled to form a high 
estimate of the vast number of these planetary assemblages, 
which our universe must contain, to exhibit one of these me- 
teoric scenes in the course of a century. As nearly all the 
temporary stars finally disappeared, the great catastrophes 
associated with them must have transpired in dark systems, 
and accordingly it would seem that the suns of our universe 
must be inferior in number to the dark spheres with their 
worlds revolving around them. But it must be remembered 
that, in illuminated systems, the occurrence of such events 
would not be indicated to distant worlds with the same re- 
markable appearances. Were the planet Mercury, or the first 
satellite of Jupiter, passing through their last stage of exist- 
ence, the meteoric scene would be attended with a considerable 
accession to the light flowing from the solar region ; but to an 
astronomer, in one of the worlds of Sirius, the event would 
be manifested only by a slight variation in the brilliancy of 
the star which our sun formed in his firmament. 

Could we obtain any definite information respecting the im- 
measurable age of worlds, we might be able to form some more 
correct idea of their numbers in space from the rate at 
which they drop from the stage of being. But from our ig- 
norance of the density of the inter-planetary medium, we 
cannot estimate its effects in destroying motion, or in setting 
limits to the duration of the several members of our system. 



CONCLUSION. 131 

It may, however, be safely asserted that a million of centuries 
is only a very small part of the age of the planets of large 
systems ; and it is only in such that the fate of a world could 
be attended with such displays of light as were witnessed in 
1572 and other times. We may, therefore, be assured that 
the mysterious power to which planets owe their existence has 
not been confined to any limited portion of our universe, but 
that its impartial action has been manifest in supplying not 
only luminous but even dark systems with retinues of worlds. 

In our own system, the power concerned in planetary form- 
ation has been displayed in giving attendants, not only to the 
sun, but to a majority of his large planets. It would seem that 
Mercury, Venus and Mars have not been supplied with any 
secondary bodies ; but it is even doubtful whether these worlds 
are too small to obtain the government of other spheres. The 
devastation of time is felt more by the secondary than by pri- 
mary planets ; and during the innumerable ages which Saturn 
must occupy in entering the region where Jupiter now revolves, 
many of his satellites will have passed away, and it is doubtful 
whether he can have more than two or three remaining. Even 
these must disappear before he comes as close to the sun as the 
earth or Venus is at the present time. It would, therefore, be 
rash to conclude that Venus never had satellites, or that the 
earth never had more than a single one. In the chapter on 
Saturn's rings, I have given reasons for regarding this great 
annular collection of matter as the ruins of two satellites which 
the resisting medium of space consigned to destruction, by 
slowly reducing their orbits until, at last, they were compelled 
to revolve too close to the central body, and to experience, to 
a fatal degree, the dismembering effects of its powerful attrac- 
tion. 

It was the opinion of Aristotle and his followers, that the 
heavenly bodies are absolutely immutable in their character 
and eternal in their duration. Similar ideas respecting plan- 
etary existence have been reproduced in modern times, and 
many astronomers have contended that the condition of our 
solar system, and the relation between its members, will con- 
tinue during all eternity without any permanent alteration. 



132 CONCLUSION. 

So great was the partiality shown to this doctrine in France, 
during the last century, that the fall of meteoric stones was 
generally discredited, as it was found difficult to reconcile the 
fact with the eternity of planetary motion. But while meteoric 
falls show that the inferior attendants of the sun are continually 
dropping from the stage of cosmical existence, the appearance 
of temporary stars may be considered an indication that even 
mighty worlds are not exempted from catastrophes which bring 
their planetary career to a close. The division of Bielas's 
comet, and the present condition of the asteroids must be 
considered as indications of the violent convulsions which oc- 
casionally disturb the tranquillity of celestial operations. Even 
in the absence of any premature disaster, the resisting me- 
dium which the planets pass through must preclude the pos- 
sibility of their eternal duration, and the ring of Saturn presents 
an example of the condition to which satellites will be reduced 
in the course of time. 

It is not to be supposed, however, that these changes will 
destroy or impair the grandeur of the vast domains of crea- 
tion. In every department of nature the work of renovation 
always keeps pace with destructive agencies, and we might 
reasonably expect that there is some provision for counteract- 
ing the devastations of time in the heavens. Although the 
formation of planets does not seem to come within the scope of 
scientific inquiry, it has often afforded a favorite subject for 
speculation to the astronomer and the geologist. According 
to the nebular hypothesis, as maintained by Herschel and 
Laplace, the several planets were thrown off as rings from 
the sun, at some remote time, when his matter was so much 
rarefied by heat that it filled the greater part of the solar re- 
gion. But these views have been gradually abandoned by 
astronomers, and the few facts on which they depended have 
been successively withdrawn from their support, until, at the 
present time they can be only considered as purely speculative. 

Though the origin of worlds, like many of the operations 
connected with life, may baffle the researches of exact science, 
we may trace the career of these bodies through ages of in- 
conceivable length. From their continual, though impercepti- 



CONCLUSION. 133 

ble approach to the sun, it would seem they must have com- 
menced their existence on the verge of the solar system, and 
that their vast age comprises the time they take to reach the sun by 
insensible approaches to him at every revolution. During such 
an immeasurable age they must experience a great diversity of 
climate as they successively pass through the cold, the temper- 
ate, and the warm regions of the solar system. Regarding the 
career of planets in this light, the climatic conditions of each, 
and the purposes which they are capable of fulfilling, cannot 
be as different as might be first supposed ; and the researches 
of the geologist . may throw much light on the present, the 
past, and the future condition of the several planets. All 
worlds like the earth appear to have their stages of prepara- 
tion, of utility, and, perhaps, of decline ; and as we may look 
on one man as a type of the human race, so our globe may be, 
to some extent, considered a type or specimen of the number- 
less worlds of our own and other systems. 

Whatever doubts may hang over all speculations respecting 
distant events, either of past or future time, we have reason to 
believe that our universe will ever exhibit great and useful op- 
erations throughout its extensive domains. From the ruins of 
some celestial bodies others will arise to act a part in the drama 
of the physical creation in future ages. Though nature's works 
may all deeay, her laws remain always the same, and numer- 
ous agencies, obedient to their control, and aided by occasional 
interventions of creative power, must maintain the heavens 
forever in a harmonious condition, and transform innumerable 
spheres into seats of light and intelligence. While the laws of 
nature have been thus wisely ordained for such great ends, 
their simplicity renders many of them intelligible to the lim- 
ited powers of the human mind, and the immense universe 
thus becomes a vast field of intellectual enjoyment for man. 



APPENDIX. 135 



APPENDIX. 



THE FORM OF MARS, AND ITS INFLUENCE ON HIS CLIMATE. 
(See Page 36.) 



Many circumstances, which are apparently too trivial to attract the 
notice of the astronomer, have a very decided influence on the domestic 
conditions of the several planets. Though mountains cause only an in- 
appreciable deviation of our earth from a spheroidal form, yet they have 
a very great influence on the climate and fertility of surrounding lands, 
and on the geographical distribution of rain. But a far more decided 
effect must result from the form of the planet Mars, which has a flatten- 
ing about ten times as great as his rotation is capable of producing. On 
page 36 I have regarded this as an indication that the planet must have 
lost much of his rotary motion since he assumed his present form. But 
there is still better ground for tracing the influence of his form on his 
climate, and on the position of his lands and seas. 

According to the measurements of Arago, there is a difference of 128 
miles between the equatorial and polar diameters of Mars, while the 
measurements of "William Herschel indicate that the flattening is con- 
siderably greater. "Were the planet fluid, or composed of readily- 
yielding solid matter, the equatorial diameter could not be more than 
twelve miles longer than his axis, since the relation between gravity and 
centrifugal force is nearly the same as it is on the earth. But, though 
the solid frame-work of our planetary neighbor is too stubborn to obey 
the laws of equilibrium, the liquid and gaseous materials which roll 
around him will submit to their influence, and, accordingly, he could 
not have anything like an equable distribution of air and water over 
his surface, unless his rotation were performed in seven or eight hours. 
In his present condition the principal oceans must be situated around 
his poles, and here, also, his atmospheric covering must be far more 
dense and extensive than between the tropics. 

Now, the density of the air has a very great effect in increasing the 
heating power of the solar rays, and, accordingly, in elevated localities 
the temperature is exceedingly low. An unsupportable heat is always 
felt at the base of tropical mountains, while their summits are the seats of 
perpetual snows ; and the physical geographer must regard the torrid, the 
temperate and the frigid zones as existing, not only on the earth's surface, 
but in the different strata of the atmosphere. Could the tropical regions 



136 APPENDIX. 

of the earth be elevated seven or eight miles above their present level, 
they would be, in all probability, much colder than those places near 
the poles, which would acquire a more dense covering of air by the 
arrangement. 

If we can depend on the measurement of Arago, which Humboldt con- 
siders worthy of confidence, we may reasonably conclude that the torrid 
zones of Mars occupy his circumpolar regions, while the lands of his 
equator must experience the rigors of an eternal winter. The bright 
patches around the poles of this planet are generally regarded as collec- 
tions of snow, which ara deposited during winter and melted at the 
approach of summer. This is supposed to account for the fact that the 
area of the brightness diminishes at each pole in proportion as it is ex- 
posed to the more direct action of the sun. But, as William Herschel 
has shown that clouds in the atmosphere of a planet cause a superior dis- 
play of brilliancy, the fact alluded to may be regarded as an indication 
that the polar oceans of Mars are covered with clouds during winter, but 
as summer approaches the vapor suspended in the atmosphere is 
rendered invisible by heat, and the cloudy region will be diminished. 

According to Lyell, the principal oceans of our planet were located 
at the poles during the age at which the mammoth lived. This arrange- 
ment, he supposes, gave our earth a much higher temperature than it 
now enjoys. But if, as we have reason to believe, the great size of the 
polar oceans was due to a more considerable flatness than the earth now 
exhibits, the difference between the climates in different latitudes would 
be much less than it is at the present time. If the difference between 
the equatorial and j)olar diameters were thirty-two miles instead of 
twenty-six, while the length of the day remained the same, very little 
difference could be expected in the mean temperature of different zones 
of the earth's surface. 



ASTRONOMICAL EVIDENCE OF THE UNIVERSAL DELUGE. 

(See Page 37.) 

There has been much diversity of opinion in relation to the last geolo- 
gical revolution, to which allusion has been made on page 37. At one 
time the universal deluge was supposed to be the great cause which en- 
tombed all organic remains in the earth's crust, but nearly all modern 
geologists contend that no general flood ever took place. A very differ- 
ent opinion has, however, been maintained by Cuvier : " I agree, there- 
fore," says this profound writer, " with M. M. Deluc and Dolomieu, that 
if anything in geology be established, it is that the surface of our globe 
has undergone a great and sudden revolution, the date of which cannot 
be referred to a much earlier period than five or six thousand years ago, 



APPENDIX. 



137 



that this revolution overwhelmed and caused to disappear the countries 
which had been previously inhabited by man and the species of animals 
now best known ; that, on the other hand, it laid dry the bottoms of the 
last seas, and formed of it the countries at present inhabited ; that it is 
since the occurrence of this revolution that the small number of in- 
dividuals dispersed by it have spread and propagated over the newly 
exposed lands, and, consequently, that it is since this epoch, only, that 
human societies have assumed a progressive march, that they have 
founded establishments, raised monuments, collected natural facts, and 
invented scientific systems." 

This doctrine has, however, been opposed on the ground that a 
general deluge cannot proceed from any known operation of natural 
causes. The idea that the great insurrection of the seas was caused by 
the sudden upheaval of the circumpolar lands has been prevalent for 
some time, but the causes which might produce this elevation were 
pointed out in an essay on the subject, which I published in 1856 ; I 
also showed that the great number of lakes in high latitudes may be 
regarded as a proof of the late upheaval of these lands ; for, as lands 
grow old, their lakes must gradually disappear, from the influence of 
rivers in filling them with sediment and excavating channels to draw 
away their waters. I shall here present the reader with an explanation 
of the manner in which the upheaval took place, as it was given in the 
publication to which I have referred. 

Hitherto the most reliable evidence of the recent elevation of lands 
has been derived from the occurrence of ancient sea margins, at a con- 
siderable height above the present level of the ocean. On the coasts of 
Scotland, of Sweden, and of Canada, geologists have recognized the 
ancient boundary between the land and water ; and they have, there- 
fore, inferred that these regions have risen above the surface of the ocean 
at a very recent geological period ; while the great number of lakes they 
contain leads us to the same conclusion. This elevation of lauds in 
high northern latitudes, seems to have taken place on a very extensive 
scale ; for not only British America, but the northern part of European 
Kussia, and probably a large part of Siberia, were submerged during 
the tertiary period. On the other hand, the observations of Mantel, in 
New Zealand, and of Darwin, on the coasts of Patagonia, show that a 
similar rise took place in the corresponding localities of the southern 
hemisphere, at about the same geological period. There are also some 
indications of a recent depression between the tropics ; and it appears 
that, previous to the advent of man, the earth had a greater amount of 
land in the vicinity of the equator, while its polar regions were occupied 
with deeper and more extensive oceans. 

That such was the condition of our world at the close of the tertiary 
period, is the belief of most modern geologists ; and the warm climate 
which then prevailed in high latitudes, is ascribed, by Lyell,to the vast 



138 APPENDIX. 

extent of tropical lands and the great size of the oceans encircling the 
poles. Such an arrangement, as this able writer has shown, would have 
a very decided influence in elevating the temperature of all climates ; 
whereas, if the great continents were located about the poles, leaving 
the equatorial region to the exclusive possession of the water, the cold 
would become sufficiently intense to render much of the land unin- 
habitable. 

There is some difference of opinion respecting the last geological rev- 
olution, which elevated so large an extent of the polar regions, and gave 
our present continents their geographical outlines. Some maintain that 
it was effected by a vast number of elevations and depressions, contin- 
ued for many thousands of years, during which the boundary between 
the land and sea varied at the same rate as in modern times. Others 
contend that a sudden rise of land took place, over the whole or a very 
extensive part of the polar regions, driving their seas to other localities. 
The watery invasion arising from this upheaval they regard as the 
cause of the dispersion of the drift, and it has also been identified, in 
some respects, with the great flood which almost annihilated the human 
race. 

Before deciding on the physical possibility of so extensive a catastro- 
phe, it may be necessary to direct our attention to a cause which, though 
generally overlooked, is not without its influence in the affairs of our 
material world. From recent astronomical discoveries, it has been as- 
certained, that the planets do not move through an absolute vacuum ; 
the realms of celestial space being filled with a rare fluid. The exist- 
ence of this medium was first indicated by its effects on the revolution 
of Enpke's comet ; but it is from the alteration which it produces on the 
size of Saturn's ring that we derive the most conclusive proof of its 
dissemination through space. The inner edge of Saturn's nearest ring 
is continually approaching the surface of the planet, at the rate of about 
200 miles annually ; and the orbits which its several parts describe 
are thus slowly decreasing in magnitude from a diminution of their ve- 
locity. The change could only arise from the resistance of an setherial 
medium, which acquires unusually great density in the vicinity of so 
large a planet. In other parts of space it has such veiy great rarity 
that the change which it produces on planetary motion, during many 
thousands of years, could not be detected by the most delicate astro- 
nomical observations. 

Without examining the other proofs which astronomy furnishes of the 
existence of this rare fluid, I shall proceed to trace its effects on the con- 
dition of the earth in past times. There are, indeed, some slight indica- 
tions that it has produced a considerable change in the dimensions of 
the earth's orbit since the Silurian strata were deposited. At this period, 
according to most geologists, a uniform temperature prevailed in all lati- 
tudes ; and accordingly the greater part of the heat must have been de- 



APPENDIX. 



139 



rived from the subterranean regions ; while comparatively little was 
supplied by the sun. It would seem, therefore, that during this palae- 
ozoic age, the sun was far more distant than at the present time ; and 
that our planet has drawn considerably closer to the central luminary 
during the many myriads of years comprised in geological history. 
These conclusions, however, can have no pretensions to infallibility, as 
climate does not depend on the sun's distance alone ; and there is much 
uncertainty in the information which organic remains furnish, in regard 
to the temperature of the primitive world. 

We shall, however, meet with more success if we endeavor to trace, 
in the occurrences of past time, the effects which the medium in ques- 
tion produces by impeding the earth's rotation. As the earth's diurnal 
motion is slowly retarded by this impediment, the centrifugal force ex- 
periences a corresponding decline ; and the waters in the equatorial 
seas, having less to maintain them in their present position, must grad- 
uallv withdraw to the poles. If, for instance, the day increased twenty 
minutes in length, a reduction of three per cent, would take place in the 
intensity of centrifugal force ; and the waters at each pole would rise to 
a height of nearly 1400 feet, while the seas at the equator fall about 700 
feet below their present level. The solid parts of the earth would also 
have a tendency to rise at the poles, to sink at the equatorial region, and 
to assume the arrangement dictated by the new relation of gravity and 
centrifugal force ; but their cohesion would prevent them from yielding 
to the impulse, and for a long time compel them to retain their old posi- 
tions. 

There is, however, a limit to the strength of the terrestrial fabric ; 
and the cohesive force on which passive strength depends, exerts less in- 
fluence over masses when their size becomes great. "Were our globe com- 
posed of wrought-iron, its immense size would render it incapable of 
preserving its present form, after its diurnal motion had entirely ceased, 
and centrifugal force no longer modified the effects of its attraction. Now 
the component parts of the earth's crust are much inferior to iron in 
strength ; and, according to most geologists, the greater part of its in- 
ternal region is in a fluid condition. If this molten mass were inclosed 
by a crust three or four hundred miles in thickness, it might be prevent- 
ed from rising at the poles, until the day had increased twenty or thirty 
minutes in length. But though the internal fluid were prevented from 
obeying the laws of equilibrium, until most of the water had emigrated 
from the equator, to form deep and extensive polar seas, the increasing 
strain would finally cause the crust to give way ; and the longer the 
rupture was deferred, the more awful would be the consequences. As 
the bottoms of the Arctic and Antarctic oceans were suddenly elevated 
by the pressure of the internal lava, the waters which have been accu- 
mulating in these regions for many millions of years would be com- 
pelled to make a very precipitate retreat to other parts of the earth. 



HO 



APPENDIX. 



One of these mighty revolutions which, though of very rare occur- 
rence, are yet unavoidable from the circumstances in -which our earth is 
cast, seems to have occurred at the close of geological history. It has 
been long the prevailing opinion of geologists, that the drift deposits 
were produced by an extensive irruption of the sea, the destructive in- 
fluence of which was felt in every part of our terraqueous globe. The 
character of the drift, as compared with other formations, is, indeed, a 
sufficient indication that it must have been dispersed in a very tumult- 
uous condition of the waters, and in a scene of violence unparalleled in 
the history of our planet. While several facts indicate an oceanic inva- 
sion of the land, on a most stupendous scale, there is also evidence that 
the supply of water was derived chiefly from the polar regions. The 
pyramidal terminations of Africa, South America, and the southern ex- 
tremities of Arabia, Hindoostan, and Malacca, have been made the sub- 
ject of speculation by some writers on Physical Geography, and are as- 
cribed to a great flow of water from the south pole. But the most reli- 
able evidence that a vast deluge of water burst from the polar regions 
to overwhelm the land, is to be derived from the abundance of the drift 
in high latitudes, and its continual decrease as we approach the equator. 
Bowlders, also, occur in the greatest numbers in places remote from the 
equator, and entirely disappear between the tropics. Of these strange 
rocks many may have been cast in their present situations during the 
diluvial commotions ; but it is probable that the greater part were sub- 
sequently distributed by the agency of ice, during the intense cold 
which began to prevail, when this mighty catastrophe gave new geo- 
graphical positions to the land and water. 

As the greater part of the polar seas were converted into land, while 
much of the equatorial lands were overflown, the temperature of the 
earth's surface would be much reduced ; and many regions would ex- 
change a mild for a frigid climate. Lyell contends that terrestrial 
climates have experienced such an alteration in recent times ; and he 
ascribes it to a like cause ; but he supposes that the change in the 
positions of oceans and continents took place by numerous upheavals 
and depressions, continued for many myriads of years. This opinion, 
however, is inconsistent with the doctrine which most geologists (in- 
cluding Lyell himself,) hold, respecting the agency of ice in the dis- 
tribution of the bowlders. The age at which these were distributed is so 
closely connected with that in which the mammoth tenanted Siberia, that 
geologists have been unable to fix any line of demarkation by which they 
can be separated. "Unless both ages include the same time, the transi- 
tion between them must be very sudden. ISTow, it would be impossible 
that the mammoth and rhinoceros could enjoy a mild climate in the 
vicinity of the ■ arctic circle, at the time when the central and southern 
regions of Europe were immense fields of ice, and when icebergs were 
making incursions almost as far as the tropics. The perfect state of 
preservation in which the flesh of these animals has been occasionally 



APPENDIX. 141 

found, can only be accounted for by supposing that they were frozen up 
immediately after death, and that the succeeding summers could not 
supply a sufficient amount of heat to produce putrefaction. 

Since the above was written, my researches have shown that tidal ac. 
tion is another cause of retardation to the earth's rotary motion, and I 
have removed the apparent difficulty presented by the investigations of 
Laplace, as may be seen from the fourth chapter of the present work. A 
little reflection will show that, as the rotation of the planets changes with 
time, a corresponding alteration in their forms becomes necessary, and, 
if their constitution be similar to that of the earth, this cannot be effected 
without causing a most violent convulsion of their waters. 

Professor Dana supposes that, at the most recent geological period, there 
must have been a very great elevation and a subsequent submergence in 
the region occupied by the British Isles, and that the changes also ex- 
tended to the neighboring localities. He regards the wide estuaries and 
friths, so common in these places, as caused by the submergence of val- 
leys, which were previously excavated by the rivers of more elevated 
lands. I have, however, shown, in pages 31 and 32, that the influence 
of the tides augments the eroding action of rivers at their mouths, and 
that wide estuaries are to be ascribed to their agency. The results must 
have been more decided in these regions at a time when the rivers passed 
through more lakes and introduced less sediment to the ocean. For ex- 
amples of the formation of deltas in tideless seas and of estuaries where 
the tides are high, the reader is recommended to consult Lyell's Princi- 
ples of Geology. 



ORIGIN OF METEORIC STONE8. 
(See Page 77.) 
It is now more than twelve months ago since I first advanced the 
opinion that meteoric stones are projected from the dense part of comets, 
which sweep very close to the sun. When my views on the subject were 
first given to the public I supposed that they were original, but since 
writing the article on Cometary Catastrophes, (pp. 71-81,) I have found 
that similar ideas are given in the article on falling stones, published in 
Rees' Cyclopedia. It would seem that the writer of that article was in- 
duced to ascribe the origin of meteorites to cometary bodies by a consid- 
eration of the moderate explosive force required to project bodies beyond 
the range of the feeble attraction of such masses. 



142 APPENDIX. 

SIZE OF METEORITES. 
(See Page 87.) 

The pamphlet alluded to on page 87 was published in the early part 
of 1854, and copies of it were sent to several members of the American 
Association for the Advancement of Science previous to the Washington 
meeting. My views in regard to the supposed existence of gigantic me- 
teorites are given in this little work in the two following paragraphs, 
which I insert as they originally appeared : 

The most remarkable phenomenon attending fireballs is the extraordi- 
nary disproportion between the actual size of the solid matter they con- 
tain and the enormous magnitude they seem to possess while in a lumin- 
ous condition. The fact is, that the aetherial fluid they encounter is ren- 
dered luminous several hundred feet around them ; and mechanical prin- 
ciples show that the pressure it receives from bodies moving so rapidly 
should operate through a range equally extensive. It is on this account 
that fireballs, containing a few feet of solid matter, are so suddenly trans- 
formed into globes of light half a mile in diameter, while pursuing their 
brilliant career through the firmament. The pressure which the aether 
receives from the descent of very small bodies is nearly equally intense, 
but of brief duration, as their velocity is quickly destroyed; and this 
may serve to explain the appearance of shooting-stars. The transient 
light which these objects emit might be produced by the fall of bodies 
below the size of hailstones ; and to this we may ascribe the moment- 
ary duration of the light of shooting-stars, and the failure to detect, in 
the great mass of the atmosphere, any substances wriose arrival from 
external space is announced by their appearance. 

The opinions generally held, in regard to meteoric light, surpass in 
extravagance the most fanciful doctrines of the ancient or the middle 
ages. By some it has been ascribed to the inflammation of some gas 
which encompasses the meteorite ; but no appreciable quantity of gas 
could be confined around bodies of such feeble attractive power, and, 
certainly, none could be retained by them after their terrific encounter 
with the air. To account for the apparent magnitude of fireballs, it has 
been supposed that they are solid masses, two or three thousand feet in 
diameter — and the weight of even some of the less magnificent class of 
aerolites has been estimated at six hundred thousand tuns. These enor- 
mous masses are supposed to revolve around the earth or the sun in 
orbits adjusted with such remarkable delicacy and precision that they 
never strike any part of our planet, except the verge of its atmosphere, 
and then escape with the loss of a single fragment. It is unnecessary 
to refute the idea that such bodies, traversing space in every direction, 
should always avoid a direct encounter with our planet, and select the 
narrow path which alone could save them from destruction; or that they 
could come so frequently into such perilous proximity to the earth with- 
out being, in a single instance, precipitated to his surface. 



APPENDIX. 143 



AURORAL AND COMETARY LIGHT. 

It may, perhaps, be supposed that the luminous appearance of the 
tails of comets and the phenomena of the Aurora Borealis are an evi- 
dence that the great luciferous processes of nature do not always require 
the presence of great masses of matter. But a little reflection will show- 
that even in these cases there is no independent chemical or light-pro- 
ducing action. It is the solar rays that call forth the electric currents 
that produce the Aurora, and it appears also that the long trains which 
attend comets are due to their influence. Without the sun's illuminat- 
ing agency, neither cometary nor auroral light would be manifested; and, 
even in the present condition of our system, the light from these two 
sources makes ouly an insignificant part of that which flows from the 
solar orb. 

The continual manufacture of celestial light on a small scale may be 
pronounced impossible. A globe as large as the earth, or even as Jupi- 
ter, could not be luminous except when a great planetary meteor revolved 
very close to its surface ; and, as such meteoric scenes must be ex- 
ceedingly rare in our universe, we may reasonably conclude that the 
several fixed stars must be several thousand times larger than the globe 
we inhabit. The author of the Plurality of Worlds contends that the 
matter of the fixed stars may be excessively rare, and this constitution 
will render them unlike the &un. But in this case they must have a far 
more enormous magnitude, for the size which bodies require, to be per- 
petually luminous, must be great in proportion as their component parts 
have less density. This I have shown on page 99. 



ERRATA. 



On page 4, line 5, for "contribution" read "contributions.'' 
On page 20, line 12, insert "tuns" after 6000,000000,000000,000000. 
On page 22, lines 24 and 25, for "it would have a centrifugal force," 
read "the centrifugal force would be." 

On page 24, line 18, instead of "eight," read "eighty." 
On page 28, line 5, for "are" read "is." 
On page 44, line 20, for "serves" read "may serve." 
On page 86, line 30, for "previous to their arrival at the ground," read 
"in the upper region of the atmosphere." 
On page 108, line 30, expunge "it." 

On page 111, lines 11 and 12, instead of "and the laws of equilibrium 
must evidently exist in the sun, his tornados, etc.," read "and as the 
same laws of equilibrium must evidently exist on the sun, his torna- 
dos, etc." 



144 INDEX. 



INDEX. 



I. The Influence of Magnitude on Stability 5 

II. The Doctrine of Gravitation 15 

III. Theory of the Tides 24 

IV. Effects of the Tides 31 

V. Cases of Excessive Tidal Action and Planetary In- 
stability 38 

VI. The Rings of Saturn 43 

VII. The Supposed Influence of Satellites in Preserving 

Planetary Rings 48 

VIII. Movements of Comets 52 

IX. The Tails of Comets 60 

X. Mass and Density of Comets 66 

XI. Cometary Catastrophes 71 

XII. Phenomena Attending the Fall of Meteors . . 82 

XIII. The Origin of Solar and Meteoric Light 91 

XIV. Variable Stars and the Sun's Spots 103 

XV. Temporary Stars 112 

XVI. Electrical Light and the Aurora Eorealis 119 

XVII. Conclusion. 122 

XVIII. Appendix , 135 



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